On PC: Generally around 2000 to 0200 is best for weekdays, starting earlier and finishing later at weekends, in Deathmatch mode. I tend to get in within a few minutes at those sort of times, though there's a decent chance of a match from 1800 onwards.
If you want Team Deathmatch that's rarer - we don't generally try to start it unless we have at least the minimum six, which means the other two slots go pretty fast and you're back to trying to find another five players to start a second match or waiting perhaps for two or three matches to go past (at 15 minutes each) before a space opens up.
Either way you'll find it easier to find a match if you join the unofficial discord server -
https://discord.me/elitedangerouscqc
On Xbox/PS4: unfortunately the community is a lot smaller and regular play times are less established. There are quite a few players on those platforms on the discord server so you might be able to round up a match. Probably the same sorts of times - overlap of EU and US timezones - are likely to be best.