It took me about 3 1/2 hours to get all the coins I needed. Thrall's coin was one of the rare golds, but I did get a total of two of them by the time I was done.
After the first hour of fishing, I only needed 5 coins: 1 copper, 1 silver, and 3 gold. (A friend of mine reported the same pattern). After another hour of fishing I got the missing copper. After another hour I got the missing silver, and two of the missing golds. The final gold came up about 1/2 hour later.
Here are some suggestions:
-- Throw back your gold coins (one at a time!). During the 2 minutes that the luck buff lasts, you should get seven catches. 2 to 4 of these should be coins.
-- Get your fishing up so that you don't have any fish "getting away", as this wastes your luck buff. Here are some ideas to increase your fishing skill:
1. Use the best pole you can get, but use the +25 Pagle pole as a minimum.
2. Use a +100 lure (a good source of these is the Outlands daily fishing quest)
3. Go to the Dalaran sewers and search for Underbelly Elixirs. If you are lucky, drinking one of these will turn you into a walrus guy with +50 fishing skill.
4. Use your cooking skill to brew up some Captain Rumsey's Lager which will give you another +10 skill.
5. Get the +5 fishing enchant for a pair of "fishing gloves".
These measures alone will boost your skill by 190, which should be enough to avoid the "your fish got away" problem for anyone with a skill in the upper 300's.
Of course, you may also have other "+" goodies that this site can tell you about, but the one's I've listed are probably the easiest to do.
-- If you haven't already done so, bind a key or a mouse button to the casting "spell" so that you don't waste any time re-casting once you've clicked on the float.
-- Get out your favorite pet, clean it so it sparkles, and feed it a pet treat so that it loves you. This, of course, has absolutely no effect on your fishing skill or chance to catch anything, but it does make you feel better, and you can always toss a ball of wool for it to fetch every so often to help break the monotony... :)
Hope this helps and Good Luck!