I have a Rheem rooftop heater / ac that is 13 years +, the ac works really well during the hot summers,
When fall hits and we start using the heater, the house heats up really well, but the problem is when the heat stops blowing the fan contiunes to run for 10, 15, 30 min to a hour.
It varies.
Is their a timing switch or something that can be ajusted, or what needs to be replaced.

There may be a feature that permits the fan to run a bit after the heater element is turned off (so as to not waste the warm heater element). The Model number of the unit and a little internet research will confirm this. Copy and paste the following address into your web browser:

http://www.rheemac.com/products/ProductInfo.aspx?XSL_File=rheemac_products.xsl&Market=Commercial&SectionVar=Cooling&Category=Condensing%20Units

That style of Rheem packaged unit uses a combination fan limit control switch. That style of switch is common to stick and was used on all brands of heating equipment. It's a part that can be picked up from any hvac supply store but you need to know the length of the fan/limit control.