Giving back to alma mater
Why are you concerned about expectations?  After you get your parchment, your school no longer has anything to give to you.

What you give is purely from your own generosity -- because you feel by giving back to the school, others will benefit.  You give because you feel it is the right thing to do.  You give because you want the school to purchase that lab upgrade.  You give because you want to fund this research because you think it's important.

"Expectations" from an alma mater are not only irrelevant, but it is wrong to think that you owe them something.  You paid tuition, they gave you your parchment.  That deal is orthogonal to "donations" and post-grad support of alma mater.