Turn that amp down!
OSHA recommendations state that you can listen to 90 dB levels for 8 hours a day. Every 5 dB increase halves your safe exposure time.
To get a ballpark estimate of your exposure, assume that your speaker has an efficiency of about 100 dB/watt (most modern guitar speakers are close to that figure) and that most of that power is making it to your ears (a reasonable assumption in the small spaces most of us have to play in).
Every doubling of amp power adds 3 dB. Multiplying amp power by 10 adds 10 dB. A five dB increase results from (approximately) tripling your amp power.
So what you do is figure out the dBW (watts expressed as decibels relative to 1 watt; so a 1 watt amp is putting out 0 dBW) of your amp and add it to the dB/W efficiency of your speaker.
Speaker efficiency depends entirely upon the design of the speaker. The most common modern speakers have close to either 97 dB/W or 100 dB/W sensitivity. A few vintage-style speakers (the Jensen P10R, for example) are closer to 94 dB/W. Several modern speakers (the Eminence Tonker, Beyma Liberty 8, Fane AX12, ...) are closer to 103 dB/W. The only way to know for sure is to consult the manufacturer's literature.
Here's an example... Say you have a 10W amp with a 100 dB/W speaker. The amp is putting out ten times one watt or 10 dBW. Add that to the speaker efficiency and you get 110 dB SPL in the room. That's four 5 dB increments above 90 dB SPL, each of which halves your 8-hour safe exposure time. So you can crank that 10 watt amp, but you're going to (according to OSHA) start accumulating hearing damage if you do it for more than one-half hour per 24-hour period.
A cranked thirty watt amp through the same 100 dB/W speaker will start to damage your ears after just fifteen minutes.
A cranked hundred watt amp through the same 100 dB/W speaker will start to damage your ears after just seven minutes!
A fifty watt amp is only about 2 dB louder than a thirty watt amp, and 3 dB quieter than a hundred watt amp.
Also keep in mind that many amps at full clip will put out an additional 3 dB or so above their rated power. That gives the 10 watt amp the potenial or a 20 watt amp to damage your ears, which cuts the safe exposure time by about 3/2. In other words, that same cranked ten watt amp through a 100 dB/W speaker will damage your hearing after only twenty minutes.
There's no way to know for sure the full power output of your amp without measuring it. There are lots of things that affect full-power output: output transformer losses, power supply sag (due to both rectifier tubes and loose vintage-style filtering) and cathode biasing all limit your amp's full power at clipping. The power at clipping will never be more than 3 dBW above the full undistorted output.
Earplugs reduce your exposure. 20 dB earplugs give you four 5 dB increments of reduction, so will increase your safe exposure time by sixteen times!
If you're playing with a band your best bet is to not guess: measure the room SPL using a meter. You'll be surprised at how loud a band can be in a rehearsal space unless everyone makes a conscious effort to play at a very low volume. You can get a decent SPL meter from Radio Shack for about fifty dollars; every electric guitarist should have one.
Finally, you need to be aware that exposure is cumulative. There's a complex formula for integrating the amount of exposure at all the different levels you encounter during the day. If you run your amp at half-power for a certain period of time, then you've "used up" a certain portion of your safe exposure time for the day. Don't make the mistake of thinking that you have to worry only about full-power assaults; lower-powered operation also used up your safe exposure time, only at a lower rate. Keep that in mind and err on the conservative side when you're doing rule-of-thumb calculations like I've outlined above.
Wear those earplugs, folks! Etymotic Research sells universal 20 dB musicians earplugs (one size fits all) for about twelve dollars a pair. I keep a pair in my pocket at all times. And turn your amp down! Partial deafness is about as rock `n roll as you can get, but folks like Pete Townshend will tell you that it's a lot more fun getting there than being there...
What can you do (aside from turning down your amp) to limit hearing damage? There are several ways to reduce delivered power.
If you're running an amp which has four output tubes, you can pull one pair of tubes to get about a 3 dB reduction in output power. If your amp is cathode-biased, consult a tech before pulling tubes; many such amps will cook the tubes if you pull one pair.
You can use a Hot Plate or other attenuator. I have never heard an attenuator that sounds acceptable if you use it to bring the amp down to conversational levels. But that's not what this application is about. If you knock off 5 dB - something that most attenuators can do without adversely affecting your sound - then you've doubled your safe exposure time.
One more thing you can do is to use low-efficiency speakers. Here, your choices are limited. But the right combination of amp and low-efficiency speakers can sound wonderful. The speakers in my Vibro-King(s) are rated at about 94 dB/W. That's 3-6 dB down from the (apparently) typical 97-100 dB/W speakers now in common use. So my 60 watt amp pushes the same SPL as a 30 watt amp with 97 dB/W speakers or a 15 watt amp with 100 dB/W speakers.