HVAC systems are essential to any home in this day and age but especially in Florida where the heat and humidity get so high. It is not unsual for the humidity to get up into the high 90s.
Heat pumps or an air handler plus air conditioner are the most prevelant systems in Florida.
I have seen HVAC systems last for 25 years however I have also seen them go after 8 years especially if there has not been routine maintenance. The system should be serviced at least once or twice a year. Not doing this and not changing the filter which becomes dirty restricting air flow can significantly decrease the life span of an HVAC system.
The other component of a HVAC system is the ducting. Ducting does not last forever and some say to replace them every 15 years or do. Over time they can also begin to leak and ducting typically should be inspected once a year. Ducting should be cleaned every 3 to 5 years and if one has pets or is sensitivie to allergens it may have to be done more often.