If the fluid level doesn't show on the dipstick when COLD, it is definitely a full quart low. It is difficult to "read" dipstick right after adding fluid, so make sure when transmission is cold, you have at least added a half quart- drive car a bit to warm up and get a good reading after new fluid drains off the stick.

It will run for years if you watch level and add as necessary. I have had a new trans (84k) miles waiting to put in but transmission in the car is still perfect after leaking 5 years from front seal. Hope that helps.