Ok, IŽll bite.
You first add the fuel flow per hour of all the engines You intend to feed from the tank in question. Let me call this figure FF [kg/h] (substitute mass units as required).
Then You determine how much fuel You want burnt off the tank. Let me call that amount M [kg].
The time required to burn up M, assuming no power changes occur in the meanwhile, will be:
M divided by FF equals time required [h]. Multiply this with 60 to achieve the minutes required.
Of course, if You have no CrossFEED, but a CrossFILL installed with no published rate (as in the DH8), there is no real substitute for keeping an eye on the pumping process...