Inherited thousands of Magic Cards
Your first goal is to separate bulk from value. Bulk cards are usually valued by the inch (for example, they'll buy for $0.20 an inch, which works out to about $0.005 per card) so getting value cards out of the bulk pile is paramount in properly valuing a collection.

The first thing you'll want to do is sort the cards by set. This will make them much easier to look up. I suggest you do this with the cards upside down if you're right handed so it's easier to thumb through them. Remember that cards will naturally have some order to them, so you will likely have a lot of runs in the same pile. If you find there's too many sets, then only pick out a couple of sets per pass (ideally the ones that have the most, to reduce the amount of picking on the next pass). I sorted a collection of 10,000 cards in five passes, but I used my knowledge of the block structure to help with that.

There's a very nice guide to identifying sets on this site.

Once you have them sorted by set, you'll want to go through each set and find the value cards. Go to a website of your choosing that lists the prices by set, and filter by the price so you only see prices over a certain threshold. (When pricing collections, I use $0.75 as my threshold, since they're the ones likely to pop over $1.00 on occasion...)

Separate your commons and uncommons into one pile and your rares and mythics into another pile. This can easily be done by flipping the pile upside down and thumbing from left to right. Commons are black, uncommons are silver, rares are gold and mythics are red. Now go to your price list for the set and identify if there are any commons or uncommons over your threshold. Don't let the Lightning Bolts and Ponders get lost in your bulk pile! Next look at the number of rares or mythics in the set on your price list - if it's like 8 or less, just find those 8 by name in your rare pile. If it's more than that, sort your rare pile by name, and sort your price list by name, and go down both the list and pile at the same time. You can skip large chunks of the set this way, and you'll find that sorting by name gets a lot easier after you've done it a few times.

I find on average it takes around 1 hour per 1000 cards using this method. You end up looking at each card 4 times doing this:


Separate by block (so you only have about 20 or so piles)
Separate blocks into sets (one pile into four piles)
Separate into common/uncommon and rare/mythic
Picking out commons, or picking out /sorting for rares


So a 20,000 card collection should take around 20 hours or so to sort.

This doesn't take into account foil cards. As a general rule, just to save time, any full art card, foil card, or dual-mana producing land I will set aside and look up separately.