At what point do you feel comfortable taking a hero you've been practicing into comp?

I've been training a lot of tracer lately since she's slightly more useful after the update, and lately I've been looking at what I need to still do before I can take her out of qp and custom aim lobbies. I'm curious about how you guys look at it; do you compare stats with other players, shoot for a certain number of hours, or just go by feel?