Italian engines will go 10-12 gallons with proper care. The only way this will happen is if you change the engine bearings as needed. Bearing life depends on ABEC and CLASS rating. The higher the better usually. Ceramics are king for all engines. They rotate much freere and truer. A new conrod will be needed around 6 gallons roughly. Most recommend changing the conrod after breakin, which can take 1-2 gallons. breakin is really rough on the rods. Novarossi, Axe Rossi, Picco, JP are a few examples. These are race engines, with high compression ratios and are started with a bump start box, which can be rough on the engine bearings if not done properly. These also turn 40K rpm's! So add all that up, and you see why they need bearings and rods even though they are made from better materials and better manufacturing tolerances.
Yor basic RTR engine won't last as long. Sometimes they do, but it's very rare.
They are not held to very high manufacturing tolerances, no two will run the same or last as long. They need beariings immediately after breakin IMO. The bearings are such poor quality that some are worthless out of the box. Axial is one I feel is a good example.
I have seen two Axials last 9-10 gallons, but these were ones the factory actually got right. i had 4 that ran hot and performed poor, one of those the main bearing fell aprt during a 1 gallon check. I got lucky, if I had ran it one more tank it would have been a complete disaster.
Biggest factors are proper breakin and tuning. Good fuel with a heat cycle breakin is the keys to an engine that astounds the average basher's expectations. I have had several 100 dollar engines that with no modification would kill 300-400 dollar engines, just because I have done my homework and had many good tutors along the way.
Traxxas engines are throw awys IMO. 4 gasllons is the average life span with good tuning and no parts replacement.
Save your cash and buy an O.S. 21 TM and enjoy a nice running quality engine.