The chance of getting free-spins on Munchkins is once in every 93.87 spins.
I don't know what it is on Avalon, but I would guess it's in the 140 to 160 range.
KK
The reel data is below, which can be used to determine the rate of bonus rounds or the chance of 5 wilds. At first glance, it looks like the reels have 34 symbols and 1 scatter per reel, which would make the chance of 3+ scatters is 1 in 167.
1st set:
<Reel symbols="0,7,5,10,2,9,7,5,10,2,9,11,8,4,10,8,7,5,10,2,9,1,6,4,3,6,1,8,4,6,1,8,3,9" />
<Reel symbols="0,6,9,4,1,8,5,7,2,10,3,9,4,1,10,5,11,10,3,7,6,2,10,6,8,5,9,4,1,8,7,3,9,2" />
<Reel symbols="0,10,5,9,2,8,1,3,10,5,9,11,4,6,8,1,3,10,4,7,6,2,9,7,8,3,6,8,2,10,5,9,7,4" />
<Reel symbols="0,9,8,4,6,5,7,2,1,7,9,8,4,6,5,10,3,9,8,4,7,11,6,10,3,6,10,2,9,10,1,8,5,7" />
<Reel symbols="0,10,8,5,6,11,7,9,3,6,7,9,3,1,10,8,5,1,4,2,9,8,2,4,10,8,5,9,10,1,9,10,2,7" />
2nd set:
<Reel symbols="0,7,10,5,9,7,8,11,4,10,12,6,10,5,8,2,9,7,8,4,6,3,9,7,8,2,10,6" />
<Reel symbols="0,6,9,4,6,9,4,6,9,4,8,7,10,2,3,10,5,11,10,7,8,12,3,8,9,5,8,7" />
<Reel symbols="0,10,5,9,3,8,12,10,5,9,6,11,9,4,6,7,4,8,2,10,8,3,6,8,2,10,9,7" />
<Reel symbols="0,9,8,2,9,8,2,9,8,6,3,10,6,7,11,4,7,6,4,7,12,10,9,5,8,10,5,7" />
<Reel symbols="0,10,8,6,10,8,6,10,8,6,3,11,7,9,12,4,9,5,4,7,9,2,8,9,7,2,3,7" />