I have possibly never loved a question so much - because I have wondered a similar thing sometimes when making plans with adults!! (Invite a friend to do X next Tuesday, no firm committed reply, how long to wait until it's not rude to move forward on plans with a different friend, etc.)
I will be interested to see the responses!
In my opinion, for a child, I think the best answer is something along the lines of: "okay - but if I do not have definite plans with you by around 2pm, my mom wants me to go with her to my aunt's house", something like that. The child can "use" parents as the "bad guy" excuse for moving on without the first friend thinking that the child did anything wrong.
(The first friend in your example probably either had immature-casual disinterest, or, was an eager friend who just needed to check with parents. Either way, it's fine to put a time limit on that.)