I started my lo very young before 1month, maybe abt 1week. i started putting her on my chest first. once shes stronger, i put her on the bed for tummy time. i start for few seconds first. she loves to be on her tummy during her playtime. now shes gg 2months soon, she can support her neck quite well. even the nurses was surprised hahaha her current tummy time duration from 4-10mins each time and will do her own rollover. i recommend daily as it helps with strengthening neck muscles and so on. no. jard and fast rule. as long you know when your baby is tired and put baby on the back.
You can actually start doing tummy time from 1 month old. 3 month old baby should have at least some neck strength already.