There is a entity called "area_effect_cloud" (AEC) which is used for splash potions. It is very handy for this kinda stuff since it has an "Age" tag. It despawns when it is 1 tick old (a 20th of a second). So you can easily control the life duration. If you summon that entity with an age of for example -20, it lives 21 ticks (or 1.05) seconds before it despawns.
So if you summon an AEC with a certain age on a block, and let all the AEC aged 0 remove their blocks, you can make a trail.
It is a bit more complex than that, since you also have to think of making the trail bigger when it eats an apple, but I think that's the best way to explain :).
