Originally Posted by OzExorcist
As much as you think your opponent will call.
That's a different amount for every situation and opponent though so as ukaliks and c9 have pointed out above, the real answer is "it depends".
Originally Posted by only_bridge
2. You dont always want your opponents to call.
both these statements are correct.
On one hand, you want your opponents to call especially on a dry board, so based on your opponent, you have to make a bet that you think will be called.
On the other hand, you don't want your opponents to call a small bet when the board is draw heavy(flush/straight), so in this instance you want to make a bet that will give opponents improper odds
to call, thus forcing them to make a mistake in calling.