It's tough to come up with a useful average. For one, there are multiple versions of Adobe Garamond out there. You have to factor in four main variables:
- Character width
- Letter spacing
- Word spacing
- Glyph frequency (relative to your normal content)
The first three get really fuzzy if justification is involved. For general purposes, some people rely on the average width of the alphabet but that's very rough.
You would need to check the metrics for the printed version of Adobe Garamond that your end product uses vs the fonts you have available in your digital reference system. You're probably better off getting a TypeKit license and putting the right font in your app.
This old site lists average character widths in standard type 1 units (1000) for a number of fonts, including Adobe Garamond. For the regular weight, the average for all characters is 497. Search in the page for 'AGaramond' if you'd like to see other info.