can only give you some info i have from our sop and fcom. carbon brakes efficiency increases with temperature. that's why it shouldn't be a problem to take off with (very) hot brakes. but when the gear is retracted these hot wheels come very near to the hydraulic lines and that's why you've got a temp limit on t/o, ie 300° or 150° with brake fans on (because the temp probe is cooled down by the ventilator).
on landing our sop recommends brake fan on:
-if a wheel is at more than 500° (!)
-5 minutes after landing (to have enough for temp stabilization)
-before entering the stand (because of the carbon particles that are projected around)
typically at max landing weight and full use of brakes and reverse thrust you'll get to 300° about 3 or 4 mins after landing.
also in a 45 minute turnaround the brakes (without brake fan) will be down to 100° (no problem for take off) and with the brake fan on you'll need about 25 minutes to be down to 50°.
and a last comment. when taxying the best method is to let the aircrft accelerate to 25-30 kts and then slow it down to about 10 kts, let it accelerate again, etc... your brakes'll stay lower in temp than if braking constantly.
hope it helps.
seb