This is a very difficult question to answer given the variables involved. For instance, which planets are included in the alignment group? Also, what exactly are planets?
If the alignment group of interest includes the current one of Mercury, Venus, Mars, Jupiter and Saturn, then alignments happen on occasion, but the schedule is sporadic. These five are called the "bright" planets because most humans can see them with their naked eyes easily. Their last alignment happened in 2005, but the next alignment after the current Jan/Feb 2016 morning alignment will be an evening alignment in August of this same year.
If the question is about all of the planets aligning together, visible or otherwise, the question is a lot harder to answer. The current known eight planets plus the historical/traditional dwarf planet of Pluto last aligned in 561 B.C. and are expected to do so again around 2854 A.D. However, given that planets are capable of being demoted and that more actual planets may be discovered (as evidenced by recent news), it is impossible at this time to say for sure when a full alignment would happen.