For chloramine control, one hour should be more than enough, but it's really up to you since hardly anyone has a UV system on a residential spa and hardly anyone properly dosing with chlorine complains about chloramines.
[EDIT] Also, since the FC went up in your UV Off case, I presume you had the SWG on in both situations. So this means that with only having the UV on for a shorter period of time, you should be able to turn down the SWG on-time and that will have the SWG cell last longer. Given what you found with the UV rather rapidly depleting chlorine, you should consider that the UV manufacturers were touting their wares intentionally withholding materially important information you needed to know to make an informed purchase decision. You should consider that before believing at face value other claims that they make -- not necessarily that such claims are false, but may be misleading (overstated or not relevant). [END-EDIT]
[EDIT] Also, since the FC went up in your UV Off case, I presume you had the SWG on in both situations. So this means that with only having the UV on for a shorter period of time, you should be able to turn down the SWG on-time and that will have the SWG cell last longer. Given what you found with the UV rather rapidly depleting chlorine, you should consider that the UV manufacturers were touting their wares intentionally withholding materially important information you needed to know to make an informed purchase decision. You should consider that before believing at face value other claims that they make -- not necessarily that such claims are false, but may be misleading (overstated or not relevant). [END-EDIT]