15-20 is realistic starting off for F2P. I have 28, but that’s at O3-4 and with each cap raise I tend to be able to advance 1 or 2 more than what I had previously. Note that I’m not competitive at all in PVP because of lack of new meta heroes and enhancing.
Also bear in mind that you can gain VIP levels through tapjoy/fyber surveys - just a few VIP levels won’t take long and improve your experience greatly. I’m up to 7 now