As Revan mentioned, there is no set number of fights before a win, but you can still spam a HR boss to reach your maximum output of all your heroes potential. Which is usually an additional 10%-15% of what you averaging and your low will roughly be around 25% less of your average. Your not going to achieve your maximum often so continue upgrading your runes as needed.

If you know your match up's and mechanics, a simple tweak to your heroes/formation will suffice.