This festival is not large like the Phoenix festival, so you won't need much time to see everything. Maybe two leisurely hours, unless you want to hang out and listen to the live Greek music by Synthesi.
There is a mixture of Greek and non-Greek items for sale. The prices seem reasonable. I got a pack of Greek gum and was happy to see the expiration date is not until December of 2019, so the stuff is fresh, not outdated.
Check peoriagreekfest.com for more info, including a two-for-one admission coupon. There is free on-site parking; very convenient. The whole event takes place outdoors, except the church tours and historical film on the life of this church's patron saint, Haralambos. FYI (only if you are not Greek): pronunciation is Ha-RA-lam-bos. I asked. And yes, you roll the R. Anyway, I didn't go inside so I can't comment on the tour or film.
If you are in the area, drop in for an hour or two. You will be glad you did.