I've been having success with a caffeine and guarana-based fat burner. I'm aware of the downside to excess caffeine intake but how much is too much?

There has been much discussion but little agreement on how much caffeine is too much. Around 300 milligrams a day is generally considered a moderate intake of caffeine. This is equal to about three cups of coffee or six cups of tea.

Moderate amounts of caffeine are generally not considered dangerous to most people.  But remember that caffeine is a diuretic which causes the body to rapidly eliminate fluid which can lead to dehydration if not replaced with fresh water.  Another consideration is the possible exhaustion that artificial stimulation with caffeine based supplements could cause.  Stimulants such as caffeine allow the body to power on beyond its tolerated, natural threshold, pushing it into an advanced state of exhaustion which could leave you in a heap if you push it too far for too long.

There is a wide variability in how much caffeine people can tolerate before they start to notice negative side effects.  If you get the jitters from one cup of coffee, then that is too much for you. If you can't sleep after drinking a cup of coffee, then don't drink coffee in the evening.

Remember that a lack of energy is not due to a caffeine deficiency.  Caffeine is not a requirement in the diet and falls more in the category of a drug than it does a food.

A supplement such as the Life Fat Metaboliser which provides the proven fat reducing nutrients L-Carnitine and Garcinia, is more likely to provide lasting fat reduction, without the possible negative effects of a stimulant such as caffeine.