Hello,

I am currently on a non immigrant B visa which will expire at the end of April this year.

My wife is Thai. Can I apply for a marriage visa now, in other words early, or do I have to wait till my B visa runs out?

What exactly do you have now. A visa from a embassy or consulate or an extension from immigration?

If you have a multiple entry B visa you can apply for an extension during the last 30 days of any of the 90 day entries the visa allows.

If you have an extension of stay from immigration you will have to wait until the last 30 days of your extension. If your job ends you can apply for the extension based upon marriage on the date you job ends by canceling your existing extension.