Memes are alts too. They blossom whenever general alts are mooning and do not have a separate time to blossom. However, most of them have a good run when they're newly listed and have good narratives behind them. A good number of them have shown promises and have enabled those who invested on them on time to flip accounts. Nothing gives a better ROI than an investment in memes (no doubt),  in as much as it's also more likely to burn investors' funds faster than an investment in credible altcoins.

As for when their useful narratives will last, no one can truly tell that.