How long does a play through last?

Depends on your goals. If you want to become a millionaire and get the highest possible scores, you'll need around 60+ hours. If you just want the best ending, it's still probably 30-40 hours, with much of it being spent in the spring and summer year 2. However, you could beat this game in under 10 hours easily if you don't care about the ending and just want to do minimal farming and animal care. In fact, you could technically beat it in under and hour if you just keep going to bed repeatedly, but that's not really "playing" the game in any meaningful way.
---
- The Admiral

Some do's and don'ts from the FAQ. I removed the more spoiler-ish ones, so these are fairly basic and should get you started on the right foot.

THINGS YOU SHOULD DO
- Collect eggs and milk from livestock as soon as the day begins.
- Feed all your animals after collecting produce. Brush and talk to cows as well. Remember, time does not pass in the barn.
- Take care of any business in town once the produce is collected, including buying seeds or livestock.
- Collect any crops. If the day ends and you have not picked all the crops, save them for the next day. They will survive until picked.
- Collect fruit or fish from the mountain for extra money when not harvesting crops. This is especially important in the first two seasons.
- Build relationships with all the single girls in town in your free time. It is best to do this during the fall and winter when you have some down time.
- Maintain the ranch. This includes fixing broken fence pieces and pulling up weeds.
- Chop extra wood from the mountain at night. You will need to save up until you have upgraded your house twice.
- Before bed, check the weather for the next day.

THINGS YOU SHOULD NOT DO
- Plow the field, water the crops, cut the grass, or plant new seeds during the day. Save these chores for the night time. Daytime is when you should be making money.
- Place anything in the shipping bin at night. You will not be paid for it.
- Keep the livestock outside. There is no benefit to doing this and there are some serious downsides. Keep them inside at all times.
- Buy more than one chicken. After buying the first, breed the rest by incubating eggs.
---
- The Admiral