A general approximation is all you can get from what I understand-- because batteries vary. But I don't know any more than that.
In all seriousness, it appears from this Energizer data sheet that total capacity is dependent on current draw. At higher current, there appear to be inefficiencies that reduce the total capacity (probably from dissipation of heat from the higher levels of current). So it's not a simple algebraic equation. In fact, if you look at their data sheet, there appears to be a logarithmic relationship. Just looking at a few different BJFE spec sheets, the current draw is on the order of a few mA. That's off their plot but it seems to project out to around 100 hours. http://data.energizer.com/PDFs/522.pdf
For a 9V battery in a *typical* low current application, I usually use Life Hours = (C_batt/I_draw)K where: C_batt is battery capacity (C) in Amp Hours (Ah) or Milliamp Hours (mAh). Mfr's data is needed here. I_draw is load current in amps or milliamps depending on what I used for C (keep units the same). K is a derating factor based on mfr data and/or seat-of-the pants approximation. Actual capacity varies WILDLY based on discharge current, temp, age, etc. Offhand, I'd use .7 for K (wild guess - use mfr. data if possible) and then decrease it even further for "safety" if I care. That may be overly cautious as 10mA seems like a gentle discharge current for a 9v batt but IDK for sure. This will give you a ROUGH approximation but I still wouldn't trust it with your band's rep until you see what you really get in practice. EX: if a 9v alkaline's C= 565mAh and your load is 10mA: (565/10).7 = 39.55 hours.
You can certainly calculate the number of hours before a battery with X amount of mAh is depleted, at a certain current draw. But a pedal will not work right up until the battery is 100% depleted - as the battery goes dry, the voltage will also drop. Eventually, the pedal will shut down because of the voltage being too low. How soon that happens depends on what kind of circuit it is, even if the pedals were to have the same current draw. A distortion/overdrive type pedal will last longer than a delay or similar. /Andreas
I'd just put a new battery in the device in question, turn it on, start a stop watch or take note of the time, and play play play until it dies. Stop stop watch or take note of the time, do the math. You now know how long the battery will last, gained valuable practice time, or if your lucky, came up with a million dollar riff.