Retrieve flames on Snap

The flames are called Snapstreak Mode. This mode is activated when two people exchange snaps for at least 3 consecutive days (at least one snap per day).

Therefore, it only takes one day of break for the flames to disappear. The counter increases as the days go by.

After almost 24 hours without interaction, an hourglass appears to indicate that the day is about to run out.

There's no point in changing the clock on your phone, as the timestamp is managed on Snapchat's servers. The only way to get the flames back is to start exchanging snaps every day again...