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 war.
There is a hint of some emotion conveyed by the addition of "now", perhaps pride
I've done it once a day for five years now (and that is quite an accomplishment).
perhaps shame
I've done it once a day for five years now (and try as I may, I cannot break myself of the habit).
but in any case the main idea seems to me to be the focusing on "now"
as only a single moment in a series which, by subtle implication, is to
continue.
CJ