You can keep your demat account (or an ISIN freeze) locked continuously for as long as you want—indefinitely.
There is no maximum time limit, expiration date, or automatic "unlock" triggered by CDSL or NSDL. Once a voluntary freeze is applied, it remains active permanently until you explicitly log in and choose to reverse it.
However, keeping an entire account-level lock active for 10 consecutive years comes with practical hurdles:
It blocks everything: A full account lock prevents any outward movement across all holdings in that account. If you buy other stocks, receive corporate benefits that require account actions, or want to trade anything else down the line, the entire account remains paralyzed.
The better 10-year strategy: Instead of locking your whole account for a decade, apply the freeze specifically to the Mid150BeES ISIN and quantity. That specific asset lock will also persist indefinitely (easily lasting 10+ years), but it leaves the rest of your demat account free and functional for your normal everyday use.