I noticed with my polys that they find tilapia much faster than they look for and find pellets. Maybe tilapia smells stronger to them and makes it easier to find.
Less than a minute for mine. the thing is, the bichir needs to get used to the food it's been eating for it to find it fast. Sometimes they sniff(find0 it but they just ignore it. i feed mine sinking pellets and brine shrimp and so they find it in a zip!
With my pair, one notices almost immediately and finds it within 30 seconds. The second one just kind of eases on over. Number one has grown noticeably faster.