I heard that we can store RI on the counter and it will be just fine. For how long can u store icing for without it going bad? thank you

I would not store a bowl of RI on the counter since it contains egg whites.  Not worth the risk of a food borne illness.

what if I use meringue? (by wilton)

If you use the meringue powder, it will be ok on the counter for a few days, but it will start to separate.