You probably wont need to clean it every few days, just give it a good clean then just check it every few days to see how its looking if starts to build up again you can clean it again
Heres a good guide to cleaning it
http://www.theguineapigforum.co.uk/...-Guinea-Pigs-including-Ears-amp-Grease-Glands
'Step Two -*Cleaning the Grease Gland
All guinea pigs have a grease gland located at the base of their spine, roughly where the tail would be if they had one. It is harder to find on some pigs than others, but boars in particular tend to have quite active grease glands, and locating it will be made easier by the greasy circle you can feel - and in some cases see! The photos below show the position of the grease gland.
Cleaning (or checking) the grease gland should be the first thing you do when bathing a guinea pig; removing the grease will ensure the gland stays clean and clear. There have been some cases where a pig's grease gland is so dirty, it actually becomes infected – so there is very good reason to keep a check on things there. A few females have active grease glands, but it’s significantly more common for boars to have greasy build-ups here. Normal shampoo will not shift the grease; a product formulated for degreasing is needed. As unlikely as they sound, the three most effective agents for removing this grease are:
1. Washing-up liquid;
2. Swarfega;
3. Paxcutol shampoo;
4. Cetaphil cleanser.
The trick is to not lather the product up too much at first; as with very greasy dishes and pots and pans, it’s more effective if initially applied neat with very little water involved, then lathered up with a wet sponge…or in a guinea pig’s case, with wet fingers! After a minute or so of massaging the grease gland, the grease will start to loosen up and you can gently peel/rub the grease off without bothering the pig. Once this is done, you can pop the pig in the water to rinse his rear end off, and then move straight on to actually giving him a bath…'
