It depends on the quality of my hair at any given moment, and on the type of hair care products used. As soon as my hair has been coloured, I generally need to wash it once every two days. When my colour starts to fade, my hair can get oily, and therefore I wash on a daily basis. In summer, best to wash on a daily basis. Of course when swimming or after any intense, sweaty exercise - I wash there and then regardless.
It all depends on your hair. If it get greasy quickly, then you should wash it more often, if it doesn't, leave it longer. There's no set frequency that works for everyone. I wash mine every second day, but sometimes leave it a bit longer.
If you wash oily hair too regularly, you are only exacerbating the problem. If possible, give washing a rest for a week. You will be amazed. Frequent washing simply stimulates the oil glands. I am yet to convince my significant other of this.
I have always had very fine hair but a lot of it. I have always washed my hair every night 7 days per week. If I need to go somewhere in the evening or a night I need to wash it again. If I don't, I believe it doesn't look anywhere near acceptable. Nowadays my very fine hair has become very thin, but I still need to wash it every night. Occasionally, If I am not going out in the day and I'm going to be doing housework or whatever, I will skip the wash for the night before. But when I do, I can't wait to get into the shower and wash it that night.