After level 20, my experience meter says that Motes of Light are awarded for further exp gain... but it doesn't say how much is needed. How much experience is needed for a post-level 20 Mote of Light? Is this amount fixed, or does it change as I earn more Motes of Light?
Go to here and punch in your gamertag and it'll show you the exact amount in number against the actual bar.
Motes of light will be granted when a player has earned enough experience to "level up" again. You will notice if you have the Destiny app on your phone or view it on the computer on the bungie site that there is an experience bar that does not reflect what your light level progress is. That is your experience bar, when that fills you will receive a mote of light as a reward. Just one way they keep you remotely interested in leveling up still.
This is the page for it on the Destiny wiki: Mote of Light
Nobody knows the exact amount yet, but you can see about how much you need by hovering over your level in your character screen. (The white level "20" not the yellow "light" level). One way to try and figure it out would be to get a bounty, wait until you fill the bar up and get your note of light, then turn the bounty in. Try and gauge about how far the bar has now filled up in comparison to how much XP you earned from your bounty. Hope this helps.
every 80,000 exp
according to http://destinystatus.com. I'll note that I've seen estimates between 35–60k and that 80k is on the high end, but I haven't seen this number contested since that site went live.