How to change Pro Sites membership period of time?

Hello, first let me congratulate the team of WPMU DEV, because the products looks very good.

I want to change the membership time of 1 month / 3 months / 12 months to other period of time. For example 1 year / 2 years / 3 years.

How to change it? Which file/code i need to change?

Thanks in advance.