Starting a fast flower strain but want grow her outdoors. If I start her next week would be able to shorten her cycle by a month or so, and therfore keep

No, fast flowering are photoperiod plants. They will flower once the nights get longer and not by age.

depends on your global location. If I understood the question correctly. Cannabis is not like a tomato plant where they have fruit in an averaged advertised amount of days, lets say 90days from seed. Cannabis is a photoperiod plant meaning it need a light cycle change it initiate flowering. Even if your growing a fast version, it still needs 12/12, lights on/off, to flower. It builds up flowing hormones at night and when its mature it will bloom. If you live on the equator and your days are always 12hrs long then yes. you can do what you are thinking as the light is always close to 12/12. But if you live anywhere else and your days are longer then 12hrs. it will not flower till it has aorund 12hrs of continued darkness. sometimes 11 works or even 10. Any cannabis seed that is a photoperiod is normally advertised by how long to take to finish flowering in weeks, You can veg it as long as you want, so if your referencing the time to harvest based on advertised weeks from the package. You might have mistaken what it is saying as its not always clear. Fast versions normally take off a week or two on that flowing time. as they ripen a bit faster, veg time can be indefinite depending on length of day. Some of the newer fast versions that are mixed with autos will turn out to be an auto vs fast, luck of the draw. The older faster version seeds was not mixed with autos and its more of less a characteristic of the strain or was a cross with a strain that had it. Autoflowers on the other hand will advertise how long it is from seed to harvest. often in days like 90-120 days as they don't rely on a light change aka a photoperiod change. Good Luck!