Bearings that are used in spinners is not actually designed for spinners but instead are widely used in skates and skateboards which are why they usually come with dust covers and are greased. Bearing lubrication plays a vital role in the life of bearings when they are under significant stress and essentially are there to separate moving parts in order to minimize friction and prevent corrosion. But because bearings in spinners are not under pressure and most likely will not be in contact with water the lubricant is unnecessary and just slows down the spinner. So what I suggest is to get rid of the grease altogether as that usually improves spin time at least a couple of times.
Remove grease by washing with soap
Soap and water are the most accessible method as you just need soap, warm water and some thin object like needle or paperclip which you can stick behind the dust cover to remove it. If you damage it in the process it’s not the end of the world as they are not necessary if you use caps on your spinner. Once you have removed dust cover you should see small balls. Wash them thoroughly with warm water and soap, ideally, you can use an old toothbrush to clean it even better. Once it’s clean you can use a hairdryer or compressed air to dry it, I do not recommend just leaving them and allowing to dry on its own as bearings are very precise mechanisms and even a tiny rusty spot can ruin them. When you have dried it you can put the dust cover back on and place it in your spinner.
By the way, if you have 608ZZ type bearings then covers will be on both sides and you will have to remove both of them, for 608Z there is only one cover and 608 comes without covers, so depending on what type of bearing you have it might be less work for you.
Bearing in above picture use metal balls but not always they are made out of metal, the material depends on what type of bearings you have, you can read more about bearings here.
Like in the method with soap you have to pop off the bearing shield to access the inside of the bearing. I didn’t have WD-40 so I used brake cleaner which essentially is the same thing and works as good as WD-40. It’s not advisable to use WD-40 in the living room of your house if you do make sure you put down a lot of towels and open a window. This is best done in the garage, on the porch, or in a basement.
Hold the bearing over a towel, and point the brake cleaner towards the inside of the bearing where the balls are placed and blast the grease out of it. Aim around the edges of the bearing, try to spray into every edge you find. Once you feel like you’ve cleaned the bearing enough spin the bearing in your hand by holding the inner race and spinning the outside, clean the outside of the bearing with a towel to get the excess brake cleaner off. You’re done! Put back the dust cover and put the bearing back in your spinner.
I like this method more as it completely eliminates the possibility of rusting and actually will remove rust if there is any, also you don’t have to wait until they dry, you can just pop it back in your spinner and start spinning.