How do I correctly write someone has been doing this for this certain amount of time? For example:
Next 30 years he wrote multiple books.
or should I write it:
He wrote multiple books in 30 years.
It can be informal
During the last 30 years he wrote (or has written) ...
For the past 30 years he has written (or has been writing) ...
Over the last (or past) 30 years, he wrote (or has written) ..."
Your choice will depend mostly on whether his writing is on-going or has ended.
Also, "multiple books" is borderline redundant. If you wish to imply that he was prolific and wrote a lot of books, then say "a lot of books" or "many books". The term "multiple books" doesn't really carry any additional meaning other than a number of books greater than one.