How long to accelerate to nr light speed from orbital speeds then decelerate back?

I don't know, but you are mixing your measurement units.

You say you want to figure out the flight "time", but then ask if a light year would be enough for that acceleration.

A light year is a measure of _distance_ not time.

If by some near magic-technology you could accelerate at 1 g for 0.946 years (346 days), you would travel 0.5 light years and reach 75% of light speed.  That's 346 days from the travelers point of view; it would be 1.1 years back on Earth.  Then another 346 days to slow down and stop, and 692 days for the return trip.

However, you would need 184 x 10^15 joules for each kilogram.  At current prices, that would cost about 16 TRILLION dollars just for the energy alone, and that's for each kilogram (2.2 pounds).  That is more that the production of the entire USA for a year.  Good luck with that.

Even with advanced technology, I have a hard time imagining a human traveling like that with less than 2000 kg per person.  So a solo trip would cost the entire economy of the USA for 2000 years.  Maybe energy will be cheaper a thousand years from now .. or maybe not.

P.S.  The "rocket equation" doesn't have to apply, because you don't have to take your fuel with you.  Ground (or space) based laser beams could provide the energy; but can you figure the power required to push a ship at 1 g?  And the tight focus to hit a ship half a light-year away?

All this, and more, indicates why "practical" travel to other stars will be at very low accelerations, like 0.0001 g's or less.  But then the round trip takes 400 years, instead of less than four years.  And that means more supplies, so a heavier ship.  The cost comes down to 55 years of the entire USA economy.  (Or we could wait 1000 years, then the relative cost would come down to where somebody in the top 1% could afford it.)