I usually replace a Shimano shifter when it no longer works and the "WD-40" trick doesn't bring it back. Seriously, when it's completely worn out. Shifters are the most expensive part on a bike, and unless you're upgrading to 9 (if your Sora is 8) or 10-speed, it usually isn't worth it.

Sora to Microshift is only a small upgrade- I'd say my Microshift shifters are only slightly better than Sora in fit and finish and action, not as good as 105, don't know in comparison to Tiagra. They're the cheapest option for 10-speed, but they still have two levers like Sora - although the downshift lever is in a better location than Sora. They have a rather loud click when shifting, but they've been perfectly reliable for me so far (1000 or so miles). I have the 10-speed shifter shifting 9 cogs using the "alternate cable routing" method, and have found this to be just as reliable as my 9-speed Shimano drivetrains. Their ergonomics- how they feel when riding on the hoods- is about like 10-speed Shimano stuff, which is pretty good- better than the 9-speed Shimano shifters, IMO. Check back after a couple of years, and I'll be able to tell more about their longevity.