@saikatdas0790: The exact maximum value has been increased to 30 days.
We are aware, that there are many use cases for which delegations without expiry would be helpful. However, these are problematic from a security perspective. So what we will most likely implement in the future are short-lived delegations that can be refreshed.
Unfortunately though, this is only a plan so far and I cannot give you a timeline of when this feature will be ready. The most up to date information can be found here: Internet Identity Roadmap Update, June 2022