Why does my online cart empty and how to combine logged in and out carts?

My customers use their cart kinda like a wishlist so when a sale happens, everything is in there ready to go. I noticed if they add stuff when logged out, that stuff doesn’t combine with a saved cart. I installed the Persistent Cart app but we are having trouble with it. My question is, how long will Shopify “hold” a cart. Does it delete stuff after a while. Is there any other solution for combining a logged out cart with a logged in cart?