Well it depends.

I know there are some ongoing roleplays that are constantly accepting members. In my opinion I like to join un-started or just starting roleplays as it gives you ample time to bond with your partners as well as to experience the Roleplay as it happens rather than jumping in and having to understand what has already happened and then trying to deal with establishing your character into the current cast without feeling left out.

Of course there are obvious disadvantages to consider. Unlike joining in-progress rp's, there is always that (very common) possibility that the RP may not get on it's feet after starting, if it even starts at all. Joining into an in-progress RP almost gurantees that you will be joining an RP that will last quite some time. 

But really its up to you. The only limit on the amount of rp's you can join, is the one you set for yourself. You could try a mixture of both! Just experiment and go meet people. The best way to test the waters is by jumping in.