Most stock silicone jobs last a month or two. Sometimes longer, sometimes less. It just depends a bit on environment, string type, yo-yo, and how you throw/bind. Everybody will have a bit different results.

I'd say order a few extras and keep them in a safe spot until you need them. You'll save on shipping and all that in the long run if you just get the extra bits when you make an order. It's not like pads go stale. Keep 'em dry and in a plastic bag or whatever until you need them.