I think the math is correct here.

A node is eligible for for selection when 90% of the other nodes have already been selected.

Once you're due to be selected, your chances are 1 in 326 (10% of masternodes, the length of the selection queue) or about:
63% in the first 13 hours (326 blocks)
82% in the first 24 hours (576 blocks)
97% in the first 48 hours (1152 blocks)
99.5% in the first 72 hours (1728 blocks)
99.91% in the first 96 hours (2304 blocks)

where probability P = (325/326)^blocks

This will shift a little as the total masternode count changes, but the probabilities will stay close to these numbers.

Now, since other unpaid masternodes are constantly being swapped into the selection queue
and that selection depends on a competition between cryptographic hashes,
it is possible your node will fall OUT of the queue before being paid. (causing a skipped payment cycle)
This is a rare occurrence, but, double payments occur about 0.6% of the time, so over time payments should average out.