there is no 'longest you can run tren'. Typically, a reasonable duration for tren ace is around 8-12 weeks. Running longer than this causes problems with recovery.
If you want to get shredded for the summer, why not run clen instead? Or on top of this? That makes more sense for your goal.
As a side note, if you want similar effects to what you were getting out of 500 tren e, I would run something like 50mg/day of tren a. It can be dosed significantly lower than E for similar effects.