The eclipse glasses contain a very special filter material that is ISO certified to be completely safe for direct solar viewing. So, as long as that material remains undamaged, you could use the glasses to look safely at the Sun anytime you wanted to. It’s not fantastically exciting to do that when there’s not an eclipse going on, though, unless:
(1) There are HUGE sunspots to be seen (this is pretty rare)
The sunspots have to be really big, because if they aren’t, then you won’t be able to see them on the surface of the Sun.
(2) There is a transit of Venus going on (and this won’t happen until 2117!)
If you didn’t see the transits of Venus on 2004 and 2012, sorry!
(3) There is a transit of Mercury going on, and you have REALLY good eyes (this does happen soon, on 11 Nov 2019)
Mercury does transit the Sun much more often than Venus, but it is very small against the Sun’s disk. Most people cannot make it out just by looking with eclipse glasses.
You cannot see granulations on the Sun’s surface, or “solar flares” or prominences, with eclipse glasses. To see those requires a filter that allows you to view the Sun in what’s called “Hydrogen-alpha” light – and those filters are expensive!
The eclipse glasses are really good for one thing – looking at the partial phases of a solar eclipse! So we recommend taking good care of them while you’re using them for the 2017 eclipse, and then safely storing them for the next eclipse you happen to see! They’ll be perfectly fine for at least a couple of years, so long as the filter material remains undamaged. Though NOTE: some manufacturers do put a 3-year lifespan on the filters. Maybe best NOT to plan to use them for 2024!)
They also make a GREAT souvenir!
So keep them safely in storage. By doing this, you’ll be doing your part to help ensure that all these eclipse glasses don’t end up in landfills! And that’s good for all of us! And if you do decide to buy “more for ’24”, they only cost a dollar or so!
P.S. Another great option has come up, and this is also highly recommended. The group Astronomers Without Borders performs great outreach work around the world, and they have announced a program of accepting used eclipse glasses for donation. These glasses will end up helping kids in less fortunate locations have a chance to view eclipses that visit their countries, and it is a great cause.
Please consider donating your used eclipse glasses to Astronomers Without Borders!