Three issues:
1. If you have been going for the full duration, you will pretty much be entering the 'I might need to repair soon zone'
2. It will increase DF times as people might just continue extending the timer, rather than being kicked out and being forced to reque with others who have been waiting for a group.
3. The vote would have to be based on all 8 people saying they want to stay, because if one person wants to leave but the vote passes because the majority wants to stay, that person has to then leave and take a penalty.

It's not a bad idea however I would argue that you already have this feature and if you want to stick with the same group, that's what PF is for. DF is your random PUG groups for those who don't want to go through the PF system for whatever reason.