When you are reviewing your MVP balance there is a number of days in the upper right. This is how many days left you have to either maintain your VIP tier or level up to a new tier. 
 
If you earn enough MVPs to maintain your tier, then the remaining amount of days is how long you have to level up. 
 
If you do not level up, then your period and MVP balance will be reset and you will have a new period in which to either maintain your tier again or level up.