An experienced Dom/Master will not push any hard limits their sub/slave has set.
When most people discuss limits they break it down further, so you have
dislikes - things you're willing to do but don't particullay enjoy
soft limits - things that you're not that comfortable with but don't mind being pushed a little, these are what i hope memories for life was talking about pushing. These are ones you often have a safe signal for so your dominant will know when they've gone too far.
hard limits - things that you will not even consider doing under any circumstances. These are ones if the dominant breaks you're willing to just walk over, possibly calling the police on the way out.
It's not difficult to set your limits, just think of the things you are unwilling to do.
But don't assume that they'll stay the same, keep talking about them with your dominant about them so they know where they stand as much as you. It's all about talking.
My sub's limits have changed considerably, there's things she set as limits that she's surprised me by asking me to do (which is a huge turn on).
It shouldn't really be a case of thinking about it each week, it's just a part of talking with your dom, discussing new things they want to try and how comfortable or not you are with them.