It's just December but I'm already thinking of Valentines party that I'll be hosting. I have some conflict in schedule but I have to deal with the other commitment as well. Fortunately, I can make some adjustments on the party but I don't want to be late for the other engagement.

I can start the party late in the afternoon or late in the evening but I want to know how long should the party last?

If you want the party to end early, you can start in the afternoon. People will understand that you will have to end the party early. But here's a better idea:

As soon as the party started, you can assign the party to another person. I'm sure someone is willing to take over to host the party. They can be your neighbor or your friend who will be there for the party. As much as possible, choose someone who doesn't drink much

I guess the answer to this question depends on the guests. If your party is for the very young ones (less than 7 years old) then the party should last no more than 2 hours. The older the guests the longer the party could be. The reason for this is due to the possible downtime that can "kill" the party. Kids can't easily tolerate a party more than 3 hours because they can easily get bored.