This is a wrong reasoning Why?
Because if you had an engine failure on take off and if you ask yourself how long time you can set FLEX N1, that means that the position of your aircraft on vertical profil is critical. Well then don't worry to much and set TOGA then you'll get an extra 10 minutes to get you out of the bush.
In other word you will be setting TOGA before having to ask yourself how long can I stay in that position. Sometimes don't think to much be basic