As to how often, most manufacturers recommend that reels be cleaned at the end of each season. I had not done that for several years, and as a result they were pretty dirty. Also, if the reel has not been cleaned since it was purchased, it may have an excessive amount of grease inside the case. The grease is their to extend the shelf life prior to sale and to help get it through the warranty period in the event it is not maintained on a regular basis. The downside of all of that grease is that it can slow the reel down and reduce casting distance. So, when you clean it needs to go.
As to the method of cleaning, here is what I have done with 3 Curados and one Diawa Team X:
1. Obtain the schematics for your reel and become familiar with them.
2. You will need a small phillips screwdriver, a small slotted screwdriver, a 10mm wrench, or nut driver, toothpicks, Q-tips, a light colored towel, reel oil, reel grease, string, camera phone, a toothbrush and cleaning solution. If you have a used dental pic from your dentist's office, they are great for working with small items.
3. There are several good videos on YouTube, watch a few before you start.
4. Alan Tani has a good article on TackleTour.com entitled "Shimano Curado Rebuild - Carbontex Drag Upgrade" which contains a series of step-by-step photos on the assembly and disassembly process. There are many others out there, but Alan's is very good.
5. A couple of thoughts. As I disassemble the reels, I photograph each step on my iPhone to aid in the reassembly process. As I remove parts I thread them through a string in the exact order that I removed them in. I clean each part during the reassembly process as it comes off the string.
6. Beneath the Star Drag, you will find a spring, nut, a washer, and two curved washers. It is important to ensure that the two curved washers go back on in the proper direction. the curves must face each other, such that you have a slight cupped space in the center between the two. Think of placing a pair of cymbals together.
7. Below the cupped washers is the Roller Clutch Inner Tube, which slides into the Roller Clutch Bearing. Be careful that you do not over oil the roller bearing as that can prevent the clutch from working properly according to the techs at Shimano. A drop or two of oil on a Q-Tip will do it.
8. On the same side as the handle is the Cast Control Cap, this is the knob that you turn to set spool tension. Inside that cap there are two tiny washers, one brass and one fiber. There are very easy to lose, and without them you cannot adjust to spool tension properly.
9. When you remove the side plate from beneath the handle, do so carefully. There are two small springs underneath that you do not want to lose.
10. Unlike Alan's tutorial on TackleTour, I want the reel clean and all excess grease removed. I accomplish this by using a cleaning solution and a toothbrush. If the Drive Gear has a build up of residue, I polish is with 1500 grit silicone carbide sandpaper.
11. I have tried Brake Kleen, Simple Green, Penn Rod and Reel Cleaner, and Rem Oil as cleaning solutions. Brake Kleen cuts the grease easiest, but it is the harshest on the plastic parts. Definitely the best for quick cleaning bearings that you have removed. Simple Green works, but does not cut the grease nearly as well. Penn cleaner is my favorite overall. Cuts grease better than Simple Green and is not as harsh as Brake Kleen. If there is not a lot of old grease that you have to remove, Rem Oil is great and it has teflon in it. Once you spray your solution on, clean with a tooth brush and Q-Tips. That includes inside the bearings and the Roller Tube.
12. Dry thoroughly using compressed air. If you don't have a compressor, you can get a small can of compressed air used for cleaning keyboards in the computer area at WalMart, Best Buy, or Office Depot.
13. Use small amounts of high quality grease and apply with a toothpick. Apply one drop of oil per bearing, and apply oil to other areas with a Q Tip. Just a couple of drops on the toothpick.
14. Reassemble using your photos.