This isn't actually how mining works. Think of mining more like playing the lottery than solving a mathematical puzzle. Following the analogy, the lottery odds are set by the number of tickets purchased – the target is to have someone win on average every 10 minutes (for Bitcoin).
Think of Slush Pool like a very large lottery pool. Let's say Slush pool is expected to find blocks on average every 2 hours, but sometimes we are just unlucky and it takes longer. If there has been 5 hours and no block found, the estimated time to find a block is still 2 hours from then, because Slush Pool is consistently buying tickets.
It makes no sense to quit because of past luck, just like it doesn't make sense to mine at a particular pool because they were lucky yesterday (or to play the lotto because 5 people won the day before).