Getting a random faerie quest at Neopets can happen every few days, multiple times a day or you could go for weeks without getting one. That's why they are called 'random quests'. I have been extremely lucky to have gotten a few Space Faerie quests as well as the Battle Faerie. If you are trying to level up a pet or add to their battle abilities then quests are a great way to go but each faerie has different rewards so you need to know what the potential outcome is before you start looking for the item that they are seeking.
I knew that the Space Faerie could be extremely generous with level increases so as long as I could find the item on the Shop Wizard I knew I was going to buy it and give it to her. The first prices I saw were in the 30K range and that was fine by me but I refreshed a couple of times to see if there was anything less. When I saw one for 13,500NP I knew I probably wouldn't find anything lower than that. There are going to be times when you aren't going to find quest items on the Shop Wizard but you can ask for help on the Neopets Quest message board. Some people have the Super Shop Wizard (that comes with a premium membership) and can find things that are selling for less than 99,999 (that is the most you can sell something for in your shop). To check out the message board, use this link: www.neopets.com/neoboards/boardlist.phtml?board=9
My level went from 205 to 209, that would have taken me four days to do at the Mystery Island Training School and depending on the Codestones I would need for each session I could cost me anywhere from 20K to 60K. I do have some Codestones saved up but some of the rarer ones are fetching 11K at present moment. I still have another 41 levels to go until I can drop back down to training every four hours but that's when things can get really expensive because you need to use the red Codestones instead of the regular brown ones.
I found the Shadow JubJub Plushie in a shop for 13,500NP so even if I only went up one level, it would have still been worth it. But wait, how can you use the Shop Wizard when you are on a faerie quest? I usually have a second window open when I am doing things at the site; when I am doing longer battles in the Battldome it helps to have another window open to hit shops or use the Shop Wizard to buy cheap items to hoard. You can search the Trading Post and go to the Auctions to find an item and if you have generous Neofriends they can send you the item that you need.
Faerie Quests can not be sold or given to people but you can buy a fortune cookie that will allow you to get another / different quest from a faerie. I have never tried this but I have to assume that you can't pick which quest redo you will get or else everyone would be picking the ones that allow you to paint your pet at the Rainbow Fountain. If I get two or three quests a week I am happy, any more than that and its icing on the cake. I used to pass on doing the quests because I was never in to training but after getting beat badly in a war I decided to train to the highest possible levels for all abilities.
One important thing to remember is the Crafting Faerie; you do not have to do her quests but you should check the Faerie Quest portal from time to time to make sure you are not on a quest for her. You can still use the Shop Wizard when you have a quest from her so if you are flipping through screens or playing games it is extremely easy to miss a quest. If you happen to get one from her and don't want to buy the items for her from the NC Mall you can decline the quest.
You can pass on any quest that comes your way but if you are looking to increase strength, intelligence, level, health points or any other attribute it is worth it to check to see if you have the item in your Safety Deposit Box or if you can find it at a reasonable price on the Shop Wizard.