This might be a shot in the dark, but here is Sheldon Brown's page which shows various makers cranks and their respective BB sizes.

What I'm thinking is perhaps you might be able to compare the Suntour crank to a similar Shimano and start there.

In any case, it seems for the triples, except for one that I noticed, the length doesn't exceed 122.5mm, so based on that, you might consider that the top end. And your estimate of 117mm might be good, but it might be a tad high.

Hopefully someone has more/better info to help you. Good luck!

P.S. I tried velobase.com with little luck. Here is the appropriate page. The page on the crank itself was of no help on spindle length.
https://velobase.com/ListComponents....7-e8e9d9083a63