Thusly "Fernando M." Spake Unto All:

Thinking about this, i was wondering what would be the survival chances
of a guppy in an acuarium with no heater?

Define "ambient temperature". As long as the temperature of the room
in which the aquarium is placed is in the 18 - 30 celsius interval,
the guppies will be fine indefinitely. If it is significantly outside
that interval, they wont be.