Mining is a chance endeavor and the probability of winning a block within a given period of time can be calculated. Unfortunately for many systems, the time required to have a good chance at mining a block can be in the months or years.
If you have several machines running at high hash rates, you will find blocks more often and may be able to absorb the variation in the rate at which blocks are mined. On the other hand if you're running with a lower hash rate and can stand running longer in hopes of getting lucky, solo mining can still make sense. If you would prefer more certainty and evenness of payouts, you will want to mine in a pool which will be covered in the next section.