Henu the Great said:
Planetary mantras can be recited for multiple of 40 days, 80 days or 90 days just like any other working, but the effect is not the same as with squares which are very potent in their own way. These would have to be started at the same time as the squares. This does not mean that workings done for example 90 days would be a bad investment. It depends on circumstances and goals. For example sometimes it is best to hit 90 days with Moon mantra than to worry about correctly executed Moon square. Slightly more room for error can give some people more peace of mind and the overall result would still be good.
I think the moon square is almost too much especially with the long mantra. In my experience you might be OK at first but this is something you grow to dread having to do after awhile also too easy to mess up or forget a day or do a day twice or something. I tried this 5 to 6 times and only had one successful one.
I think doing the planetary mantra a certain number of times for 90 days would be better.