To me "längere" feels less strong than "lange".
Wenn der DAX über eine längere Zeit steigt, geht es Deutschland gut.
The "längere" here tells me that the author is talking about some weeks.
Wenn der DAX über eine lange Zeit steigt, geht es Deutschland gut.
The "lange" tells me that it's about some months or longer.
It depends on context and subjective factors how you interprete the time spans but I'd say in general the "längere" expresses a shorter one than "lange".
Think about a similar case:
Da ist ein alter/älterer Mann.
When saying "alter Mann" I think about someone who is already older than ~ 60. When saying "älterer Mann" I think about someone who is around ~ 50.
I think this is because "längere" or "älterer" are helpful to avoid saying the very clear adjectives "lange" or "alter". They are expressing that something is close to being describeable by the corresponding adjective but not quite. This is what I mean by feels less strong – is weaker.
Maybe you can think about it like that: Someone is young, "jung", someone else is not young anymore; he is older, "älter". When you say he is old, "alt", then you don't have this fine relation to the opposite adjective anymore. From this perspective "älter" is between "jung" and "alt".
In analogy to the "Zeit" example: kurze Zeit – längere Zeit – lange Zeit