Most leagues I am in the trade deadline is about 2 weeks after the MLB non waiver deadline leaving 6 weeks or so left in the season sometimes 7 weeks depending on how the schedule is setup for MLB
I am trying a new thing in one keeper league I run where we allow normal trades commish reviewed until 8/17 but teams can "waiver" trade after that, meaning can trade all year but anyone below you can block a deal but has to be publicly blocked so heaven help you if you block someone else's then try to do one yourself

. And yes there will of course still be vetos to block, but that happens all the time in the real life waiver trading period. Teams in last and second to last can trade with each other without a veto if they wanted to shake things up for the upcoming season, and in theory other teams lower in the standings may be able to make some future adjustments.
But anyway mid August seems to work best IMO, allows some digestion to watch the real life crazy trading that can shake up jobs etc and a couple more weeks for fantasy tams to figure out if they are in it or not and what they need.
"I'm the man with the ball. I'm the man who can throw it faster than F***. So that's why I'm better than anyone in the world." - Kenny Powers