It depends... 

What was the amount of time you two agreed upon?  
Whenever I make payments on a seller's cel (or 
when someone makes payments on mine), we come to 
a mutual understanding and set up a sheduale when 
to have the payments in.

If your buyer said they would send a payment 
weekly, and a month has passed, then perhaps you 
should tell them to send the rest or the 
transaction becomes void.