ghazard,
1) This is what I tell my customers. It is not a perfect rule, but I have never had a board dry out when using these directions. Each wood act differently and may retain the oil for longer or a shorter time. :
- once a week for the 1st month
- Once a Month for the 1st year
- Any time the surface looks 'dry'
- You can buy mineral oil at almost any Riteaid or walgreen's for a low cost option.
If your board is going to see little use, then maybe you should look into a Beeswax/minerial oil finish or a salad bowl finish. Be careful with other type of oil because they may contain nut products. Remember, some people who may get/use your board could have a nut allergy and this can cause issues with their health.
2) You should always treat both sides of a project. Remember, you are changing the characteristics of the wood surface with oil. If you do not do both side, you may cause warp… Also, the oil helps protect the wood. An unprotected surface may discolor, mold, or something else. Once again, treat the board like you would any wood table top.