"Viel" corresponds to the English word "much", and "viele" means "many". In your example:,"Zeit" is an uncountable noun, as well as its English translation "time". You can't say "viele Zeit", in the same way as you can't say "many time".

In this type of expression,  "viel" doesn't change its form. There are only a few cases where it does. There is "vieles" (always neuter) without a noun, meaning "many things". Then you decline "viel" when used as an adjective together with a definite article: "die viele Zeit" - the big amount of time in this specific given context. And then there are some older fixed expressions like "vielen Dank".