as some of other people have already stated, it depends heavily on the situation.
if i had around ten big blinds or less then i would consider going all in.
if i had more than 15 big blinds then I would probably raise 3 or 4 big blinds if no one has raised in front of me.
if no one re-raises and your jacks or any pair for that matter connect on the flop then I would consider slowing it down and check/call depending on the texture of the rest of the flop.
if someone re-raises you preflop or if someone opens with a big raise then I would consider folding.