there have been three 100 ml's so far. The first two where held about 6 months apart. The third lan was held about a year after the second.
Due to the huge success of the February 100 ml, i would hope to see another in maybe 6 months. Pride might have some better details for you.
The regular monthy lans are also a great time, and they do a good job holding you other until the next 100 man.