I've thought about it for some time/I've been thinking about it for some time

[Topstudent]

I'm trying to write a dialog:

GUY: Maybe it's time that I make some changes in my life. I've thought/been thinking about it for some time. (pause) I've been working 80 hours a week for the past ten years...

Is 'thought' perfectly natural here? And is it just as natural as 'been thinking' also if you forget about the following sentence 'I've been working...'?

 

[Barque]

In that context "have been thinking" seems much better than "have thought".

 

[jutfrank]

This is not really a matter of 'naturalness'. It's a case of which aspect the speaker thinks will express better what he wants to say.

We use the continuous aspect to emphasise the ongoing nature of the action. From the context you've provided, the present perfect continuous seems like a very good choice.