Just curius what everyone else does...

If you let one sit for the winter (or had 2-4 months off for whatever reason), once you start conditioning and working to get one legged back up, how long before you will enter them?  Do you have a timeline or minimum number of rides/weeks?  How do you guage their lungs or whether or not they're physically ready to enter again?

This is something everyone will have an entirely different opinion on. Mine have sat for 5 months, they'll be ridden 2 months before running. 5x a week riding. Some people would get on and after 2-3 weeks go run. For mine I make sure we can long trot for 20 minutes without being super winded and lope for at least 5 minutes nonstop at varying speeds. I'll breeze them down the long sides of the pen or out in a bean field before it gets planted. They need to be able to sprint without breathing like they're going to die. The first week back on I just did a lot of walking and trotting, just at a sitting trot. 2nd week build up the trotting with some long trot and around a minute of loping. I just keep an eye on their breathing and push them a litte further each ride. If one minute of loping gets me 5 laps around the arena, the next day I'll push for 5.5 laps, then 6, etc. By the time I get off before winter they're long trotting half hour a day and loping at various speeds at least 10 minutes a day, 5x a week. This is just my personal preference, I like mine pretty fit. And usually the long trotting/posting is my workout for the day as well. You'll find a lot of people who think this is overboard and some who think it's not enough. This is one topic I've found that most people won't ever agree on lol 

granted  been a while but  our were out all the time in 20 and 40a pasures they be off a couple month  then long troting 2 to 3 miles  then after a couple weeks breeze up the hill about 300yards  that was us it worked for our guys but every horse different  ours played and ran in the pasture i swear they ran harder longer than we ever did but.. difference they were free

Definitely depends on if your horse is stuck in a small paddock all winter or if they're out in a big field. Ours are out in a big field in 3 feet of snow and continuously moving around and pawing at the grass under the snow. With these guys I'll get back on and do 10-20 min of long trotting for a week or so and then build them up longer and then start loping out in the fields. After about a month of this I'll take them back to running. 

I go on breathing recovery time and at least a month of good solid riding.   Mine self exercise well by playing in their 2-3 acre lot.  Yesterday went out to catch them and prior they had been running around bucking and kicking for 10 minutes before I ever left the house.  They do this multiple times a day.  That helps.

If my horses were in a stall, it would be at least 2 months of good solid riding.  

That phrase to me is 5-10 min of long trotting (build up as time goes along) and then 5-10 min loping, and then some hand galloping and breezing.  We usually long trot more because that's an overall workout. 

Everyone has their own opinions, and I think it also comes down to the individual horse. My gelding has 3 weeks more riding on my mare this spring, and my mare I feel has almost caught up to him fitness wise. 

For me, I try to get 4-5 weeks in of riding 4-5 days per week. I don't take them around any barrels until that 5ish week mark and that is usually a good indicator of if they want to work, or if they are still struggling/sluggish. My first run back at an event is usually just a cruise through at a local jackpot to gauge where we are at. 

My horses usually are done early December and I start riding again late February/early March.

I also think it can be horse dependent but my plan for this spring is...

3-4x a week for 3 weeks. Mainly trotting, some loping (mostly because we have crappy ground) and lots of walking. Get their brains back to work mode.

4-5x a week of long trotting, loping, circle drills for another 2 weeks

Then I still start doing some pattern work.

6 weeks essentially where I feel we can start doing drills and slow work. 8 weeks before i feel theyre ready to run.

When I can long trot/lope for at least 10 minutes and have them recover to normal respiration/heart rate within 5 min.

I’ll start out with trotting/loping for 5 minutes 3 x per week and increase 2 minutes per week, as they can handle it until I’m up to 14 min ( about 2 miles).

I think it depends on the age of the horse, too. My mare doesn't get back into shape as quickly as she's not a spring chicken anymore. I think I've been riding her lightly (2 to 3 times per week) for 2 mos now, and will start riding 4x per week for the next month before hauling her. 

I like for her to be able to trot at various speeds for 2 miles, lope a good 5 min without being winded before I will take her out to a race and really ask her to give me everything she's got.