Stairlifts are type tested and accredited to BS safety and also EU Harmonised EN81 standards therefore if batteries are new the lift would be capable of 10 full journeys (1 journey = Up+Down) in a 24 hour period with a new set of batteries and fully charged. Batteries should last for 3 years for the design to comply under normal charging conditions.
As you say angle and current will dictate the current draw times of availability, as will servicing of the lift and end users weight and rail length and so on.
There are many variables; however the preceding at full load and maximum angle still remains at 10 full journeys. You mentioned playing around with the batteries and voltage rates; those rates are low for a battery not under load and if that is fully charged, even lower if that is the charge voltage being applied. With a low voltage you will not have enough electromotive force to supply the required running currents. All tests would need to be conducted with new batteries as a reference to the original design and at full rated load, angle, and maximum lenght of rail which I doubt you will be able to recreate.
To answer your question of longtivity, I assume your batteries are 7Ah therefore you would be able to measure your maximum current draw from either the lift specification or through directly measuring draw using a full and new set of batteries and simply using Ohms Law to calculate design.
My experience is that you will not restore your existing batteries even with high voltage de-sulfation method and will remain to suffer. If your batteries are aged they they won’t store a full charge through natural detereation and if they have been left to discharge below 5volts they will be damaged internally anyway.
