if you have competent owners then vetoes are good. But there are owners who veto a trade that isn't worth a veto just because they made a trade offer as well and theirs didn't get chosen. Or they've dealt with either or both owners in past and don't like them so they just veto every trade they make just for the hell of it. Or the trade is being made with a team they are in contention for a playoff spot with so they don't want that team to get better so they veto it.

I'm Commissioner of all leagues I'm in and I like to have everyone involved and usually use vetoes. But I've had those situations happen before in some leagues and changed the rule and not allow vetoes.

If you do use vetoes then yes you should allow the whole time to expire. Everyone knows the rule and so if they log on on once every day they should have the opportunity to see a trade made and make their choice. If someone doesn't log on for a couple days and miss a trade that happened and think damn I would have vetoed that if I saw it oh well they lose out. But you don't want someone who logs on multiple times a day logging on 20 hours after a trade was made and see it was put through after 18 hours.

Some people don't like having the Commish decide all trades, some do. Some like having the option to veto and will use it fairly while some just veto if they think it helps their situation or just don't like the owner(s) making the trade. It just depends on who you lhave in your league. Maybe take a vote with your league owners and ask them which way they prefer.