actually for delhezis their external gills are retained with them for a longer period of time as compared to other bichirs( senegals, ornates, pollipalmas)
however they eventually will be gone. if u want to retain it for a longer time, u can keep ur specimen alone away from other bichirs, as other bichirs are always on the move nipping away ur bichir's external gills. if everything goes well, u could keep ur delhezi with external gills for as long as if reaches 7" before it starts to get reabsorbed back.
other bichirs that u shld consider if u are so into that external gill look would be polyterus endlicheri, buy the small ones can keep em alone in a specimen tank. get the farm bred ones, and see the length of the external gills grow even when its like 8", selectively bred endlicheri from indonesian farms can continue to grow large, with gills growing till up to 2" long. at the size of 10" before they slowly get absorbed back.
one thing to note is if u are keen on keeping the external gills of ur bichir for as long as possible, a tank which is highly aerated and with veyr pristine water condition is needed