There is no time limit at ''take off power'' if flex is used. There's a time limit to use TOGA power of five minutes (10 in an emergency)*. Stopwatches were often started at the commencemnet of the take off roll to accurately time the flight time for the maintenance log entry. However, the advent of ACRS and the OOOI page, there is no necessity to do this. It's not in our SOPs, yet, half the pilots I fly with still do it.
Personally, I only do it if I select TOGA.
*(These figures are for the V2500. I don't know about the CFM56.)