We just found at that you can only have a maximum of 500 maintenance periods scheduled (support can increase this for you.)  Maintenance windows don't auto delete so at some point, if you use maintenance windows at all, you would run into the limit.  In addition, if you delete prior maintenance it will modify reports and show that maintenance never occurred. 

Feature Request:

1) Remove the limit entirely

or

2) Auto-delete old monitors (after x time period) but leave the metrics in the reports.