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, where each miner owns a percentage of the total amount of tickets equal to the percentage of the network's total hashing power that they are contributing. The target is to have someone win on average every 10 minutes (for Bitcoin).
Think of Slush Pool like a very large lottery pool, and each block like a fresh round of the lottery (i.e. its outcome determined independently of all previous rounds). Let's say Slush Pool is expected to find blocks on average every 4 hours, but sometimes we are just unlucky and it takes longer. Regardless of whether 6 hours or 5 minutes have passed since Slush Pool last found a block, the estimated odds of finding the next block are the same, because Slush Pool is consistently buying new tickets for each round.
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).