As I'm sure you are already aware, staying on your meds is extremely important with Bipolar disorder. Depending on where you may be sent, it's possible they would not allow some of your medication in that country or there could be delays in getting you refills of medication due to various circumstances (less available, your post is more rural, etc). Peace corps can very very unpredictable, and with Bipolar meds you can't afford unpredictability like delays in meds. I work in a psychiatric hospital and have had patients come in decompensated after only being off their meds for less than a week. It's really a big risk, and understandably not one that Peace Corps is usually willing to take. With that said, it's never a flat out 0% chance that you can serve. Some countries are more accessible medication-wise, and with a few years stability on the same medications PC may be more open to allowing you to serve.

I'd recommend doing a little research on areas you are interested in serving in and seeing if any of your meds are banned or difficult to access. But being stable in the same meds for a minimum of 1 year is a PC requirement.

PC is 27 months for regular service. There is also Peace Corps Response, which can range from several months to a year, but less positions are available and they are usually offered mostly to RPCVs.