the time needed depends on your system (eg. how many software applications are on your old os; doing it on ssd or hdd). to cut the time shorter, you can grab w10 iso with media creation tool, make a bootable usb with it, and run setup.exe in it to do offline upgrade. you can run the same bootable usb to do a clean install of w10 later, after successful upgrade. (make sure the computer is connected to the internet during the upgrade)

before doing anything, it's safer to do a full data backup to external storage.

tbh, worth it or not, it's up to you to tell (it's really a personal preference). some adapt to w10 quickly and their systems are running fine; some have hiccups here and there and do not like it. in terms of costs, the upgrade is still free before 29 July 2016.
thanks as always!

good luck; yeah with slow internet, everything will be much slow. in such case, the better upgrade method is to download w10 iso and do it offline.

for things like os upgrades, it's better not to take it lightly if your computer is the main one for daily use. although it's not the end of the world, but disasters can be PITA. fortunately, associated risks that can lead to disasters usually can be minimized with a simple preparation and recovery plan (prepare for the worst; you can recover much more faster when bad things happen):
1. make full system and data backups;
2. download all latest and compatible drivers for new os;
3. download w10 iso and prepare the bootable usb stick.