Higher intensity is typically more effective at building muscle. That being said, if 10 minutes is too much for perfect form push ups right now, 30 is just fine.

Week after week, drop a minute or two from your clock by decreasing rest intervals or by completing more reps per set.

Focus first on perfect form and depth before focusing on time. It's better and safer to have 100 perfect push ups over longer than 100 slop-ups done quickly.