Conversation so far:
Where did Lowell Thomas begin Lowell Thomas's career?
broadcaster with the CBS Radio network.
What year was this?
1930.
What did he do for them?
delivering a nightly news and commentary program.
How long did this last?
After two years.
What was his next job?
switched to the NBC Radio network.

Question: how long did this last?
Self-contained rewrite: How long did Lowell Thomas's career with the NBC Radio Network last?