1. I have done it once a day for five years now. This suggests to me that the 5 year period is up to now. Because of the emphasis on the duration/repetition, it might be better to say I have been doing it once a day for five years now.
The addition of "now" in such a pattern suggests (to me) "at this point in a potential series of events which may possibly continue". So, to my ear, there is the suggestion that the five years will grow into six years, and then seven, and so on. The tense actually need not even be present in this use of "now":
The troops had trained for years, and they were now ready for