Cursory examination suggests that berserk lasts a number of seconds equal to the GP invested, up to a maximum of your bonus in fighting.special.tactics. Below are the admittedly few measurements I bothered to take, using MUSHclient's timestamps for the "You suddenly feel eight foot tall [...]" and "Your berserker rage departs you[...]" messages. Durations are in seconds.
|Start TS||Expire TS||Duration||GP Expended|
My fi.sp.ta bonus at the time was 215. Allowing for rounding and measuring error, it looks pretty clear-cut. I'll test again sometime, as the 216 could conceivably have been coincidental.