This is the second most asked question (see my other blog 'How long will it take to record my album?'). Again, there are several factors. First the obvious and easy to answer ones:
How many tracks are to be mixed?
What is the duration of the tracks?
Are the tracks sonically similar (same instruments, overall sound)?
This last point might be something less obvious, but is important. If all tracks feature the same or very similar instrumentation, it is easier to create balanced mixes once a good overall sound is found. This does include the use of reverbs and other effects. The more variations there are, the longer it will take. Small variations such as for example a bit more reverb in the slower songs, a bit drier in the fast ones are easy to make. Choosing entirely different sets of reverbs and delays from song to song, while possible, will take more time.
To give an example: Recently I mixed an album of a duo with a woodwind and an acoustic guitar, consisting of 18 tracks plus a bonus one recorded with a full band including electric guitar.
The duo was recorded well, the band track less so.
In addition quite a bit of editing was required.
As a result the mix of this track took longer than the mix of the 18 other songs together.
There is nothing wrong of course to have an album of sonically very different tracks. Just be aware that the mix will take more time.
How many edits are needed?
How much work is desired on featured tracks (vocals, solos, etc.)?
Sometimes decisions about edits and choice of takes can be made during the recording session. However, it is a good idea to have some time between recording and mixing sessions for creative decisions to be made. Keeping good notes referring to timings of the rough mixes I will have supplied, will make the editing process easier and faster.
What is the quality of the recording?
If the quality of the recording is poor, it will take longer to achieve a balanced sound. Sometimes this is unavoidable - for example a less than ideal live recording that has an important documentary value for the band.
It is a good idea, to consider the mix when planning the recording session. Good planning and a little additional time spent on the recording can save time and frustration (and money) at the mixing stage.
Is the recorded sound consistent, i.e. are all the tracks of the instruments recorded in the same way and in the same space?
For example if the drums tracks are recorded in different spaces and with different mics (and possibly by different drummers on different kits), each drum stem has to be mixed afresh. The same applies to other instruments as well. Some are less affected by the recording set up (DI bass and keyboards; electric guitars) but most will at least to a degree.
A recent production consisted of 12 songs almost all of which had been recorded in different locations with different musicians. This was a creative choice and the brief of the album and therefore not unexpected or a problem. The mix did take around 3 days all up.
A note about mixing tracks recorded in other locations:
I mix on Pro Tools 12 and can also open Logic X Files.
File exchange can be tricky and the most reliable way is to export audio files for each track (if you don't have them as Pro Tools session file), making sure they are consolidated - any edits including cross fades are committed - and have the same starting time. I am happy to talk you through the process.
In summing up:
A good rule of thumb is to plan for the mix to take as long as the recording.
Editing and processing (especially vocal takes) can add significantly to the mixing time.
The more instruments have been recored the longer it will take to mix. However, if the instrumentation and performance is fairly consistent, this won't necessarily add too much time.
The quality of the recording plays a large role. Some things can't be 'fixed-in-the-mix'. I usually ask for the separate audio files of a song or the whole session to make an assessment.
Using outboard gear can add character and warmth, but it is important to consider that mix reviews will take more time to set up compared to mixes done entirely ITB (in the box, i.e., using software exclusively).