At public parks the best policy is first come first serve and stay as long as you want as long as you're playing tennis. Rules that set time limits aren't fair because it punishes the people that planned ahead and got there early. People should be allowed to finish their matches or practice session.
Time limits give people permission to put the blame on everyone else but themselves. A "play as long as you need to play" policy puts the onus on the late comer to either come earlier next time or have an alternate plan. This leads to a healthier point of view in my opinion with less anger directed at innocent parties.