It is always very high for low Ib CMOS amplifiers.
AD8605 = at least 100 Megohms.
AD8661 and AD8665 = at least 1 TeraOhm
1 way to get an estimate :
Go to the op-amp’s product page , e.g.
Scroll down that webpage to "Find Similar Products"
Click the "Expand Table" button. A new window should now open in your web
In that new window scroll down to :
Add Searchable Parameters Not Currently Displayed Above
Check the box marked "Rin"
Click the "Redraw" button.
In most cases you will then see the input resistance listed in the table.
This procedure sounds complicated but it actually only takes a few seconds
(when our website is fully working and you have a good connection).