Yes, the word lifetime is a common noun, singular, abstract noun; a word for any lifetime of anyone or anything.
A proper noun is the name of a specific person, place, thing, or title; for example:
- Lifetime Entertainment Services LLC (Lifetime channel)
- Lifetime Drive, Modesto, CA
- Lifetime Photography, Franklin Square, NY
- "Once In a Lifetime", 2006 documentary narrated by Matt Dillon