At Deepbit's current size?  Forever is a definite possibility for how long a round can last.  It would currently take Deepbit an average of 4 months to find a block, assuming difficulty doesn't change.  There is mining hardware starting to come out where a single box is faster than the entire Deepbit pool.

I believe on Deepbit's system, if you change a miner to PPS, it remains proportional until the current round is over.  It's been almost 3 years since I did anything on Deepbit, but I'm recalling that little quirk in changing payment methods.

No Pay Per Share is a fixed value which is 0.00000001905289022 per share at current difficulty.  So if you have 45 shares I would think you should have a balance of 0.0000008573800599

If your using the Proportional payout method you only get paid when a block is found and then that block gets distributed to all the shares that went into that round.