Maternity clothes size question

Jenna • We had our first baby girl September 21 2015 👑 she's our world! Now we're expecting baby number 2 in Sep!!
Ok so for any moms that have bought maternity clothes before. I understand I'll probably have to buy more clothes once I get to the end on my pregnancy. But right now I don't wanna buy new clothes every few months. I'm 5.1 and now weigh 120 pounds in 16+ weeks pregnant. I'm wearing Xs tops and small bottoms. How fast did yall grown out of your clothes? Should I buy small tops now or will my Xs fit for awhile since they are kinda long on me?