You boil off rate is dependent on kettle geometry, intensity of the boil, and to a smaller extent, the humidity. If you are experiencing the 70-80% humidity we have been having here in Nebraska the past couple of weeks, the humidity may be playing a little bigger factor.
One of the fixed factors is kettle geometry. A kettle with a large surface area relative to the depth will have a larger boil off rate than a deep narrow kettle. If you switch to a different shape/size of kettle, your rate will change.
Intensity of the boil is a factor a lot of folks have problems with. You need a good rolling boil, but not so violent it splashes. A slow simmer will not give you the mechanical agitation you need nor the evaporation you want.
There is really no real formula for computing length of boil relative to the humidity. One thing to keep in mind that for a given kettle and a consistent intensity of boil, you will boil off very close to the same volume per hour every time regardless of the actual volume of wort you start with. In other words, if your boil off rate is one gallon per hour for your kettle (a pretty normal rate for a lot of folks) you will boil off that same amount whether you start with 5 gallons or 25 gallons in that kettle.
Some folks like to compute their rate as a % per hour. Not a good idea. That works only if you have exactly the same volume in the kettle every time to start. Boiling off 1 gallon from an initial 5 gallon batch is a 20% boil off rate. If you boil off 1 gallon from an initial 10 gallon batch, the percentage is only 10%. Smaller differences will of course give you smaller percentage differences, but a half gallon difference in starting volume will throw your percentage calculations off.
If you are having troubles with the length of time it takes to hit your desired gravity or volume there is a trick that I often use. Instead of doing a flavor hop addition in the last 30 minutes of the boil, I use first wort hopping. This involves simply putting hops in the kettle as you are draining your mash tun into it. This slow steeping at mash temperatures and as the temp rises as you begin your boil will bind the flavor compounds to the wort and will not boil off like the flavors from your bittering additions after the wort comes to a boil. You will need to do a little research to determine the amount of FWH to use instead of the flavoring additions. The advantage here is that you don't need to worry about the later hop additions if you have to boil longer to hit your gravity/volume. Once you get to that stage you just add your flameout addition and you are good. This is where it is handy to have a refractometer so you can quickly check a small sample.
Hope this helps a little.
Bugeater Brewing Company