When to migrate hosting?

Try a place like slicehost.com or linode.com where you can seamlessly upgrade your account over time. Both start around $20 a month, and both will let you instantly upgrade your account. That said -- if you have a good growth trajectory, you will know when you've reached the limit of your current hosting company. But honestly, the most you'd save is about $120 a year, so if your serious about your project, I'd start with a virtual server you can upgrade on the fly, it will cost you less than you spend on 2-3 months of your cellphone service (depending on plan).

Please use Ask HN: in the title of future questions. This way users can instantly recognize when a question is being asked, rather than the default assumption that this is a link.

It's just a good idea to always have a $20/mo vps unix account. Just far less headache for just twice the price of a shared host. For one thing, it will allow you to have a platform to test several versions of your app; you can run several instances of a web app and use a proxy to direct incoming traffic to one of them based on your own criteria. You have a full blow Unix environment, install every oddball development tool you want, it's just you.

A starter level VPS account costs $20 a month, which is rounding error next to the amount of engineering effort you'll expend doing a changeover on a live site. I'd move it before you start beta. If it costs you $15 a month extra times a year until you "need" the VPS, oh well, cost of doing business.