H1b petition, stamping and transfer

I have an offer from a start up in US. My name has come up in h1b lottery. I had few questions with respect to this-

  1. If the petition gets approved, how much time do I have, to get the stamp done. e.g. It should be within 6 months or a year?? I am asking this as there is huge waiting time to get appointments.
  2. If I get my h1b stamping done and while I am getting stamping done, if I get another offer from some MNC, then can I switch to another company immediately without joining the first one(startup), and directly transfer h1b for that company or do I need to work for the startup for sometime(e.g. 6 months or a year)?

You can go for stamping while your H1B I-797 approval notice is valid, generally 3 year from the start date of 1st October.