If you're mixing the powder into stock solution, you can use it immediately, and the stock solution will last about six months bottled. If you're diluting the stock solution 1:1 or some other dilution, however, I wouldn't trust it very long.
Years ago the conventional wisdom was to let any developer sit overnight. If you have hard water a bit of crud will usually precitate out when you do this. This is particularly true with print developers.
You should leave it overnight, or at least a few hours, you should not use it immediately. It takes time for a developer to settle & stabilise after mixing.
I usually wait overnight because I mix it with hot water (otherwise it's more difficult to dissolve) and I'd like it to cool down to room temperature before using it.