I have not heard from him for long or for a long time.
Can I use for long and for a long time in the above sentence interchangeably? Or do these phrases have different meaning and usage?
I have not heard from him for long or for a long time.
Can I use for long and for a long time in the above sentence interchangeably? Or do these phrases have different meaning and usage?
No, the two are not interchangeable, and interestingly, they actually mean nearly opposite things. As was pointed out in a comment, there's actually a very good thread on wordreference.com on this very subject. To summarize the most important points, though:
First, "for long" can only be used in negative statements ("not ... for long"). On the other hand, "for a long time" can be used in both positive and negative statements.
When in a negative statement:
I did not have money for long, because whenever I got some, I always had to use it right away for necessities like food and rent.
I did not have money for a long time, and I spent many months wondering where my next meal would come from.