How long is a dog in heat?

Bloody discharge usually lasts 14 to 21 days. The receptive phase averages about 9 days, within that, and can be as short as 3 days or as long as 21.

How often does a dog come into season?

About every 7 months on average. Cornell gives a range of 5 to 11 months, and the interval differs by breed.

At what age is a first season?

Anywhere from 6 to 24 months, with smaller breeds usually earlier and larger breeds later.

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Where is your dog in her season?

Enter the first day of bloody discharge and see what to expect, with the ranges.

The first day

When this season started

First day of bloody discharge

Count from the first day you saw it, not from swelling alone.

What to expect

Enter the first day and the dates appear here.

General information, not veterinary advice

This is arithmetic from published sources. Speak with your vet about your own pet.

The short answer

If the first day of discharge was 1 October, the receptive phase (estrus) is likely to start around 10 October, though it can start anywhere from 4 to 18 October. It lasts about 9 days on average. The next season is expected about 7 months later, around 1 May, and can fall between March and September.

Stages of a season
Stage	Average	Range
Proestrus (swelling, bloody discharge)	9 days	3 to 17 days
Estrus (receptive to males)	9 days	3 to 21 days
Bloody discharge in total	usually 14 to 21 days
Time between seasons	about 7 months	5 to 11 months

The maths

Every formula behind the answer, so you can check it.

Receptive phase

starts first day + 9 (range + 3 to + 17); ends first day + 18 at the averages

Proestrus averages 9 days, then estrus averages 9 days.

Next season

first day + 7 months (range + 5 to + 11 months)

Counted in calendar months from the first day of this season.

What it leaves out

It is not a fertility test. Dogs vary a great deal, and the average hides a wide range, so it cannot pick the best day to mate. Your vet can pinpoint it.
It does not use breed. The interval between seasons differs by breed, and the sources give a few examples only.
It does not say a season is normal. Call your vet if a season runs much longer than the ranges, or if she seems unwell.

Sources

LSU School of Veterinary Medicine. The normal canine estrous cycle, therio.vetmed.lsu.edu, read 5 October 2026. Stage averages and ranges.
Cornell University College of Veterinary Medicine. Dog estrous cycles, vet.cornell.edu, read 5 October 2026. Duration of discharge and the interval range.