>>> s = "This is a very long string with many many many many and many more sentences and there is not one character that i can use to split by, just by number of words"
>>> l = s.split()
>>> n = 5
>>> [' '.join(l[x:x+n]) for x in xrange(0, len(l), n)]
['This is a very long',
 'string with many many many',
 'many and many more sentences',
 'and there is not one',
 'character that i can use',
 'to split by, just by',
 'number of words']