h1b extension and change of employer

USA H1B Visa
1 
My H1B visa expires this June (I797 valid until 2015) 

When should i start applying for extension?

I'm plannin to change employer is it advisable to get visa extension done and then change employer 

or

change employer and ask my new employer to file extension
2

The employer can apply for extension at most 6 months in advance. So either the same employer can file for extension, or a new employer can file for transfer + extension (same petition). Either needs to happen before your current petition and I-94 expires.

3

Thank you Saurabh!
My I797 and I-94 are valid until 2015
only my H1 visa expires this June and as I am planning to change my employer wanted to know which would be the better way

4

So the current employer cannot file for extension as your petition is more than 6 months old. If you change employers, the new one can request 3 year petition which can run from 2013 to 2016. Later when you go for stamping, you can carry this petition and get it stamped for same duration.

If you are able to find a good employer then you should go for it now and have them get 3 year extension from today’s date. Note that even if employer requests 3 year validity, USCIS can approve it for a shorter term based on their review.