I bought a $120 Target mountainbike 15 years ago, and rode it for years, until I got a FS Mongoose (Walmart bike)
Then the old Target bike became my wife’s / loaner bike and it’s still being used today.
I recently got an 98 Trek 7000 hardtail that I fixed up and upgraded, and I love it. Sold the Mongoose. I’ve switched off to the old Target bike occasionally, and that old thing still rides pretty good. My friends are always borrowing it, and it’s never given us problems.
I will say that typical department store bikes today don’t seem to be made as tough as they were 10-15 years ago though.
As others have said, ride it until it breaks, or a deal comes along that’s too good to pass up.
You’re out there having fun and getting fit… That’s what it’s all about.