There's basically 2 kinds of memes you can make on the site.
1. Memes with established, known templates (ie, templates that are on the popular templates tab, or memes that come up when you search imgflip that already have examples of featured memes under them). If you make a meme on those, it shouldn't take more than 1-2 hours to feature, unless maybe you've written a lot of bad words or something controversial on it.
2. Memes on templates that have original pictorial content (ie, an image you upload yourself, even if just added to the meme, or templates you search on imgflip that don't have any examples featured yet). If you make a meme on those, there's really no telling how long it will take to feature, since it has to be reviewed individually by the mods. We don't know when the mods' shifts are, or how often they do meme reviews. It can take anywhere from 2 to 24 hours to feature. After 24 hours I would just give up and assume it was disapproved.
So if your meme was the second kind, 5 hours is actually not incredibly long, and isn't anything to worry about.
There's a couple of other little tips beyond those, but understanding the difference between established templates and original templates is really the main thing.
If you want, feel free to send me the link to your meme, and I'll take a look at it.