It can be stacked up to a bit past 180 days, so you can buy it 6 times for $30 dollars.

Can you double Welkin moon?

It can be purchased multiple times to extend its duration as long as the current duration does not exceed 180 days (6 total purchases).

What happens if you miss a day of Welkin moon?

Apparently, if you have Blessing of the Welkin Moon active and don't log in every day, you'll lose 90 primogems for each day that you log in.