How good is it to only pee once a day?

Not recommended. Urinating infrequently is not healthy for the bladder. If you are accustomed to voiding this infrequently, then you are not drinking enough or more likely causing yourself to hold the urine. This will eventually cause bladder damage that may not be repairable in the future and possibly require a bladder catheter for bladder drainage. Be especially careful if you are diabetic due to infections.
Not good. Either you have trained yourself to hold your urine too long or you are dehydrated. In either case, you should be drinking an average of 8 glasses of water daily, more if you are sweating. You should have to urinate every 2-4 hours and your urine should be light yellow to clear in color.