Title. I found out that I actually really enjoy playing Mercy, but I'm not sure how many hours I should put into her on QP before jumping into Comp, where Mercy is more or less a necessity, and an inexpert Mercy spells doom.

How many? 24? 48? 100?

5883455
I don't think it's a matter of hours. I think it's a matter of how comfortable you feel playing them. If you reach a point where you naturally gravitate towards a character. Or consistently pick a certain hero when you see the need for that class, I think that's when you should be decent playing the hero in comp

5883735

5883753
Just put in the time and when you feel like you're doing a good job, go comp with that hero. Basically, if you feel good with the hero, play it on comp

5883938 I feel good with Mercy, D.Va, Rein, and Reaper.

I still get completely roasted in Comp whenever I use any four of them.

Just pick heros who require little skill to be good at