?

The usual reply to "how long" has to be "Until it is charged." That doesn't help much but the accurate answer requires a seriously complex response that is tied to the battery chemistry. I am a chemist but battery chemistry is one of the most complex of all the sub-specialties and I have worked with several battery chemists and they have attempted to explain some of the intricacies and failed, with me, at least.
The practical response is to wait to charge until the little battery symbol appears then charge for 10 hr and no longer.? This is complicated by the fact that the symbol may appear when it is least convenient and you are in the middle of an activity. The response to that is "carry a charged spare and install it when the battery symbol appears".? That means, of course, that you need 2 packs and you rotate them. Nobody tells you this in the "FT60 operations course" you took.
What? You didn't take that course? Well, neither did I because, as far as I know, it was never offered. It seems we learn these things through experience - which, of course, means mistakes.
In my case, I thought the FT60 unit itself limited the charge current when the battery pack was charged. Not! The stupid wall wart at 10V+ pushes more through that battery until it overheats and deteriorates the chemicals in the cell. Uggh!
The relatively low price of the FT60 limited the features they could include and one feature left out was "smart charging" either in the unit itself or in the desk-top charging units. Smart charging involves thermal regulation and cell internal resistance monitoring, which, although not terribly complex, do require additional active electronics that were not included in the original FT60.
I have resorted to having a standard battery pack (FNB-83) and an accessory battery case/tray (FBA-25A) into which I fit 6 relatively modern 2300mAH NiMH cells so tha t when the "Battery Symbol" appears I remove the cells and charge in a smart NiMH charger that avoids overcharging. You could, of course, buy another FNB-83 and have two on hand but I was cheap and wanted to go the route I mentioned. Besides, with the individual cells you can have seriously greater capacity than offered by the 1400mAH cells in the FNB-83.