When to get a disabled parking placard?

That is the question I am asking myself the last few days. Ever since my Rhizotomy back 6 weeks or so ago, my left leg gets painfully weak and tired. Then I am left in agony for the rest of the day just for doing "normal people" errands like grocery shopping and picking up my meds. If you need to do both.....oh brother are we in for it!
So as I said, now I am finally coming to a decision about accepting that I can't "do it all" anymore. Now don't get me wrong, I just don't want one just to have one. I would like to have it for times when it gets really bad.
Also I am set for next week to get my right side Rhizotomy done. If this left side recovery is any indiction as to how it will go, I am going to need that placard. Especially since my left side is not quite healed and I have SI joint and sciatic problems. Walking is going to be an adventure that is for sure.