[Retric]

Spacecraft solar panels: ~170 W/kg and 200kw = 1lb of thrust. ~55 million miles to Mars.

Let's say 3000 lb of weight for 1 lb of constant acceleration - time to break at other end of trip. Add in a pay load we are talking ~1/5000g which still get's you 2km/s after 12 days. This would take ~300 days to get to mars. But, you could still accelerate on the trip. You would need to adjust for the loss of sunlight as you get further from the sun, but I think something like 120 day's might be possible with solar power.

Granted all of this is orbiting the sun, and you can get a little boost from a close approach to the moon etc.