How soon after I get pre-qualified for a mortgage can I get pre-approved?

Do you suggest making appointments to view homes with listing agents? I'm looking..but I do not yet have an agent because I don't know how serious I am about buying and don't want to waste their time.

I believe pre-qualifying and pre-approval are the same thing. We are in the fourth house we've purchased and found some lenders use one term, and some lenders use the other term.

If you're concerned about wasting a real estate agent's time, go to a couple open houses. The agents are already there. Talk to them, tell them you aren't certain yet, and see how they feel about helping you anyway. This gives you the opportunity to "face-to-face" see how you like those particular agents.
We did this once, and it was great. We didn't buy the house that was the open house, but we eventually bought a different house from her. When we "down size" (kids have been moving out) we plan to use her again to list this house and buy another.

Good luck.

yes

Always make sure that you are pre approved before looking for a home. 1st it lets you know much you can afford and secondlly its works in your favor when negotiating a deal with a seller

There's no real difference. Its just a conditional approval for up to a certain loan amount.