Based on everything you provided in your question, the best thing to do is for you to do at least a series of timed trials and ideally timed trials at different temperatures. There is not enough to suggest a cooking time. As for temperature, you already have a range.
While I do not have an answer to your question on cooking time, this is what I would do next time I cook sweet potato for sweetness:
- Cook a sweet potato at 160C in the oven as control
- Sous vide 12 sweet potatoes at 57C
- At 10 minute intervals, take out one potato, plunge it in ice water for 5 min and taste, compare its sweetness to the one cooked at 160C and also to an uncooked one. Repeat until all the potatoes are taken out and the last one would have been cooked for 2 hours.
- You can repeat the entire trial at higher temperatures.
You can of course do this using an oven and over a range of temperature settings. I suggest starting from around 60C, and you need to take an additional measurement of the inside temperature of every potato you take out before putting it into ice water to quench.
If your objective is to maximise enzymatic sugar conversion, then using an oven is not going to give you much control. It is just too hard to calculate heat diffusion rates and determine when to take them out of a hot oven. The most reliable method would be sous vide, seal your sweet potatoes in a bag, and immerse the bag in a 57C to 75C water bath for quite a long time (please bear with the lack of quantitative time for a moment).
Many cooking science articles focus just on qualitative descriptions with the most useful quantitative parts missing. The piece you quoted is just like that, interesting read but with no practical value. Beta-amylase hydrolysis of starch to maltose is interesting and like most enzymes, beta-amylase works at a fairly typical temperature range, nothing especially helpful or fascinating here. The most useful bit is missing, it is all about reaction kinetics, the rate of reaction and which factors change the rate by how much, that is how much time will it take to cook the sweet potatoes at the stated temperature range. All the bits that would allow a someone to calculate cooking time. The absence of quantitative parts in too many cookery science writings (there are enough pleasant exceptions) is frustrating.
Amalyse hydrolysis of starch does not happen instantaneously and it is relatively slow compared with say baking soda reacting with creme of tartar. Not only is reaction rate affected by temperature and not linearly, concentration of starch affects it too (I am unsure if maltose concentration also plays a part) as does pH. Their effects on reaction rates are easy enough to measure and calculate that you can actually do that in your own kitchen. Ok, measuring maltose concentration is hard, but tasting for sweetness which is what is important after all is very easy. Besides, sensation of sweetness is also not linear to sugar concentration. The rest involves a clock, and a thermometer.