3rd Birthday

I’m just planning my sons birthday, we’ve never had a party before so I’m wondering if anyone has advice for me. In terms of time - what do you think is a good length? Should I say arrive between a certain time and then not worry about listing an end time? I know if you say “starts at noon” people are always late. I’m probably overthinking this… what’s a good start time for 3 year olds?? We’re just doing a backyard birthday with a bouncy castle and sprinkler