When should I need to repair items to get the best effect (just not to have them not fully repaired or not to waste repair points in vain)?

Do you mean a 100% repair? That isn't possible, any repair will make you lose total durability on the weapon/armor. Assuming my memory isn't failing me, the closest you can get is a Nano Reconstructor which is 89% repair for an item. However, I would suggest saving said reconstructors for valuable items such as Adamantium Claws just as an example.

The wiki says you should wait as long as possible to repair to get the most out of it. Just make sure you don't let them break.

Also be aware that different methods of repairing have different efficiency rates (listed in the dialog box when you repair). The portable kits are much less efficient and return less max durability then the room machines so only use the kits in an emergency.