I use baking soda/acv about once a week on average, often up to 3x a week if it's been a particularly dirty or sweaty time. Sea salt should probably be applied less often, perhaps only twice a month, because it can be drying. This could be counteracted with a moisturizing solution such as coconut oil rolled on the ends of your hair if too much drying results from the salts.
Lemon juice should be applied sparingly to avoid too much bleaching, unless you're trying to lighten your color, in which case you can put it on as often as you want. Note that if you apply lemon juice directly to your hair, you should rinse it after sitting in the sun for a while otherwise the sugars in the lemon juice will make your hair sticky and dirty.
All in all, your hair is not like anyone else's, so just pay attention to what your hair is telling you. If you're washing too often it will be dry and brittle, which you can treat with a little conditioning oil. If you notice flakes on your scalp or stringy, oily roots on your locks, you should probably wash more often. Just remember to rinse thoroughly no matter what you but in your hair, because any residue left in there will likely attract more dirt!
