I'm no expert but I think the math works roughly like this:

Let T be the network's target time between blocks in seconds (10 minutes * 60 sec/min = 600 secs)

Let X be the network hash rate per second (presently ~800 000 Sol/s)

Let N be your personal hash rate per second (we'll say 45 Sol/s)

Probability of you having a winning hash for any given period = N/X

So your time to mining a block = T / (N/X)

= 600 / (45/800 000)

= ~123 days

And the reward for a block will be H*(12.5/20000)*.80 where H is the chain height at the time you mine the block. Currently 0.3195 ZEC for a mined block.