Brent Ozar has a few tips to see if you have too much memory:
Sounds impossible, right? The saying goes that you can never be too rich or too thin or have too much memory.
However, there is one good indication that your SQL Server is probably overprovisioned, and to explain it, I need to cover 3 metrics.
1. Max Server Memory is set at the instance level: right-click on your SQL Server name in SSMS, click Properties, Memory, and it’s “Maximum server memory.” This is how much memory you’re willing to let the engine use. (The rocket surgeons in the audience are desperate for the chance to raise their hands to point out different things that are or aren’t included in max memory – hold that thought. That’s a different blog post.)
If you have too much memory, I’d happily borrow a cup of RAM.