Leaving BB's in any spring magazine (AEG mid caps and of course pistol mags) for a day or two will have very little impact.
However if you leave them full and the spring compressed for say weeks, they will likely compress permanently and lose strength to push out some of the last bb's.
If you're asking because you have done this, fill the mag to full and fire the bb's out and see if you can finish all the bb's in the mag.
If you haven't done this, I would recommend that you don't
But again some mags may not be used for the whole game day so most usually have around 8 hours compressed and will work fine for months to come (will eventually have an effect but takes a long time).
They can be stretched if this happens, or replaced for very cheap.