This question gets asked ALL THE TIME! When can you run again post half marathon or full marathon? The biggest answer to this: YOU DO YOU BOO BOO!
Okay, but really, it's super variable and depends on so many things.
There are many that complete the B2B (Boston 2 Big Sur) each year which is where you run Boston marathon one weekend, and then you run Big Sur the next. On the ultra running group that I follow, there was someone mapping out a course and did 6 days of 25 miles each day. That's a marathon per day for one week. Ashley Horner just completed a charity event where she did 280 miles in 7 days. Olympic marathon runners are running 100-140 miles per week. Paid ultra runners run 200-250 miles per week. That's 28-35 miles EVERY FREAKIN DAY! I run at my very peak 50 miles per week. I've never run more than 55 in one week, and typically I'm around 40-45. I'd love to see that increase for my next marathon but it seems to hit my body hard so I just have to see what happens.
The point is: Overtraining is variable. It doesn't look the same to each person. If you are doing high volume and you notice that your next race suffers and that you aren't able to recover well, then that's something that you should re evaluate. When you come out of a marathon, you are completely beat. You want to just eat and drown your sorrows. haha! Jk. Kinda.
Some people run a shake out run the following morning. NOT THIS GIRL! Many many will run 2-3 days following just a few miles (maybe 3-4) but most will wait a full week. I did my first marathon but I didn't this time. I felt really great (the best that I've felt post marathon) and so I talked with my coach and we did some EASY EASY runs and still are doing so.
One way to see if you are recovered is to check your resting heart rate. You will find that even days following your marathon, you are still sitting at a higher resting heart rate which means that you aren't back to your baseline. I did a heart rate assessment and at 8:45min/mile, my heart rate average was 156 which for me is about 70% effort. I did NOT expect that high of a heart rate for that slow of pace for me. It shocked me but then after talking with my coach and using my brain, it's because I'm not fully recovered yet.
How can you know what your heart rate should be. This chart is a great one to see what zone that you are in and it's going to be really variable across the board what people tell you about heart rates but this one comes from the American Heart Association. IS ANYTHING EVER SET IN STONE? (I feel ya) It's best to know your numbers ahead of time and what your normal resting is and what your normal aerobic/anaerobic is so that after your races you have a better picture!
Does not being recovered mean that you can't train? I think that's something that you need to decide for yourself. Everything is on a continuum and is determined by what's behind you and what's in front of you. Do you have races coming up? I'm doing an ultra in a few weeks so yes I have to continue training. Do you have any races? If not, then you could maybe take some more time. If you do have races, what kind? This might be a great time to get in some recovery on the bike or swimming so that you aren't using the same muscle groups.
It's important though to keep your workouts easy so that you don't end up injured. Going 8:45-9min/mile for me is hard and something that I have to try to slow to. It feels as if I'm just kinda slightly picking up my feet and dragging along. It feels almost harder than going my natural 7:45 pace. Even though it feels natural to go 7:45, that doesn't mean that I should. I know my body and sometimes that takes multiple races to figure out.
If you are someone that doesn't care about the next race and you checked it off your bucket list then by all means, sit on the couch for 6 months! That's an option too! Hahaha!
I think though that it's important to not judge others for them jumping back in quicker than you. We are all different and there are some that are used to much higher mileage and their entire nervous system can bounce back quicker therefore they can begin training again sooner. That doesn't mean that they are over training but they genuinely might just be a little bit more in shape. Lastly, there are those that do overtrain and we shouldn't ignore that either. It's all about the mindset. If you are running because you feel like you HAVE TO and that you CANT GO A WEEK WITHOUT or that you think you are going to gain weight or something then yes, that's a problem. If you genuinely enjoy training, then you just want to look at all the variables and your plans and goals moving forward and make the decision for yourself when you think that should be.
With all that said, the "typical" wait time is one week and some wait 2 weeks. You don't want to wait so long that you lose all that work you put in though ;)