How long does an incomplete survey form stay active on the system...

ie: if my form tracks an onboarding process that takes 6 months will the form data be retained if it is not submitted through the six month process?

If the form is submitted to your account, the data will be retained in your account as long as you do not delete them or the submission fall under the oldest submission limit of 500 submissions (Free subscription plan allows to store only the latest 500 submissions). If you are referring to Save and continue later method without submitting the form, the session shall be stored only for 90 days. 

Hope this information helps!

Is this 90 days from initial entry or 90 days from last entry???

It is from the initial entry of the session. This is due to the incomplete submissions are stored using the date in the submissions view.

Thank you!