If your spa usage is very regular in terms of number of people and how long you soak, then you may find that your chlorine usage is consistent. However, until that point in time you should test before each soak to ensure you've got 2 ppm FC or so. Also, just because you'll have a saltwater chlorine generator doesn't mean you'll be able to avoid adding chlorine after your soak. The "boost" mode may not be enough to oxidize your bather waste. Also, if you only use the spa once or twice a week, then you'll want to set the saltwater chlorine generator to a low level to maintain a background level of chlorine so that you start your soak with around 2 ppm, but it's very doubtful that the "boost" mode will be enough to oxidize your bather waste and you'll want to check the chlorine level after a few hours, 12 hours, and 24 hours to make sure it doesn't get to 0 (or close to it < 1).
A spa is VERY different than a pool in terms of chlorine demand. In a pool, most chlorine is lost during the day from sunlight and is fairly regular unless your weather is variable. So a single setting (on-time) for a saltwater chlorine generator works well. In a spa, however, it is the bather load that determines the chlorine demand and that is VERY variable in terms of chlorine demand. When there is no bather load and no ozonator, roughly 25% of the FC is lost each day so at 2 ppm FC that would be 0.5 ppm FC per day chlorine usage. 30 minutes of one person soaking in a hot (104ºF) tub of 350 gallons would need around 3.5 ppm FC to oxidize the bather waste. So that's obviously a huge difference. If one doesn't soak every day, then the chlorine demand looks like 0.5, 0.5, 3.5, 0.5, 0.5, ... so very spiky. If one soaks every day, then it's more consistent at 3.5, 3.5, 3.5, ... so the saltwater chlorine generator could be set to handle this though you'd still need to use the "boost" mode so that most of the chlorine generation is right after the soak, otherwise the FC can get used up and get to 0.
So the bottom line is that a saltwater chlorine generator in a spa is really only useful to generate a background of chlorine in between soaks if you do not use the spa every day. In most cases, you still need to manually add chlorine after your soak unless you soak every day and the "boost" mode happens to be strong enough to prevent the chlorine from staying near 0 ppm for longer than about an hour after your soak.