I run public leagues and without a doubt the most challenging part of commishing is approving/rejecting trades.
I've been criticized by a few for approving too many trades that are vetoed but people will veto every trade if they think it will hurt their OWN team.
When there is a veto, the first thing I see is the actual players traded. At that point, I form an initial opinion. Then I look at both team's rosters and their current records. For a trade to be rejected at my site, it has to be proven that it does not help one of the teams or be obvious collusion. Trades will be rejected if a team out of contention is trading better player(s) for lesser talent.
I agree that is is not fair that you must wait for days to get a reply. It will take a day or two for the whole process and if it gets pretty heated maybe three days but a decision must be made.
Like someone said, you have the option to call. I know people call me when they want to know something right away. It is to be expected when you give a toll-free number