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

Can you buy multiple Welkin moon at the same time?

You can buy multiple Blessing of the Welkin Moons to extend the duration of these rewards, so long as your current time does not exceed 180 days at the time of purchase.

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.