If you are leaving the "charged, but un-oprimed" rifle say in the garage or trunk of your car so that it remains ambient outdoor temperature, then you should be fine for several days.

Don't leave a cover over the muzzle end, you will trap moisture within the barrel if there is a temperature change. You only need a cover on the muzzle if it's raining.

You also don't need to cover the nipple or breech plug primer port.

My rule of thumb is this:
1) If near freezing and below, I will leave an un-primed and charged ML for up to a week or so.
2) If you must bring the rifle indoors, then I will always discharge the ML at the end of the days hunt and detail clean and prep for the next day. Load fresh in the morning. If you bring a cold rifle inside, it will draw condensation and moisture and could foul your charge or worsegive rust an excellent opportunity to get started.
3) If it's very damp or raining & above freezing, I always discharge (or you canpull the charge oruse a CO2 discharger)at the end of the day, detail clean and prep for the next days hunt. Load fresh in the morning.

Regards,

Tahquamenon