How Long Does Safe Start Take To Work?

Tetra Safe Start does not work every time. It depends how it has been stored at every stage from the factory to your tank. If it has been allowed to get too hot or too cold, it won't work.
The instructions must be followed to the letter. I haven't used the product myself but I understand that the instructions include not using a dechlorinator that detoxifies ammonia for a certain length of time before using it.

But even when it works perfectly, it does not cycle a tank instantly. It can take over a week for a fishless cycle to start using TSS.
Are you doing a fishless or fish-in cycle?

It's that conversion to ammonium that TSS is not supposed to like, unfortunately, which is why you aren't supposed to do a water change for however long it is the instructions say (as I said, I've never used TSS so I don't know the exact wording). The effect does wear off after about 24 hours.

Ammonia naturally exists in 2 forms in water, toxic ammonia and less toxic ammonium. The exact amount of each form does depend on the pH and the temperature. At higher pH, more is in the toxic form so with your pH of 7.8 you will have more in the toxic form than if you'd had a pH of 6.8.

It is a balancing act, I'm afraid. Using TSS might speed up the cycle but you aren't supposed to do water changes using a dechlorinator that 'detoxifies' ammonia. This could result in the ammonia, and later nitrite, levels getting so high they harm the fish. On the other side is keeping both ammonia and nitrite low by water changes, but having the TSS not work effectively.

To start with, I'm going to give you an ammonia calculator here You use it to tell you when your ammonia levels are getting too high and you must do a water change. Test kits measure ammonia and ammonium combined; the only way to know how much is in the toxic form is by using a calculator.
You will see several boxes on the left side for you to fill in. Leave the first box as it is. Enter the ammonia reading from the tester in the second box, set salinity to 0, and enter your temp and pH. Then click calculate. The info you need is in the lower box on the right side, the one labelled NH3 concentration. It doesn't matter what your test kit shows, this is the figure to go by.
If that figure is below 0.02, do nothing. If it goes above 0.02 you will need to do a water change to keep your fish safe. Although you may inhibit the TSS, at least your fish will be safe.

If the TTS works, you may not see any nitrite. The good news is that if you do, it is less toxic at high pH. If you do get any nitrite showing up, there are ways to mitigate the effects depending on what fish you have (some species can't cope with the 'remedy'). If you do see nitrite above 0.25, let us know exactly what fish you have and if they are OK with it, I'll tell you what to do.