Not much... Depends on your Importance score but since none of us is a big whale no one earns a big amount of xem... Whoever is now harvesting is not doing that to increase his/hers wealth.
It is just an incentive to those who support the network and the fees serve as antispam measure...

With a normal stake (2,25 Mil XEM) I've been harvesting since the beginning mostly during the day and I've harvested 462 XEMs from 8 blocks and 4 of them had 0 xems (emtpy blocks with no transactions nor fees).  It is a lottery, you may find a block which will hold a 1000 xems in fees and also you may find a block with 0 (way more often).

Also your improtance score doesn't give you any insurance how often you'll find a block.. It is just your chance... I've had days where I found several blocks and today for example I haven't found one yet... it all depends...

Set up the delegated harvesting and forget about getting rich... In the end NEM is a movement with a fixed number of coins. No new ones are created... So as NEM grows so will # of transactions grow as well as fees - ergo the rewards to those running nodes/harvesting will increase as well. :-)