"The latest messages from the Panther battalions say that the Panther is currently capable of withstanding all combat conditions, and does not even have minor drawbacks. For example, the 1st tank regiment writes on February 22nd, 1944: "The Panther in its current shape is perfect for battle. It greatly surpasses the T-34. Almost all minor drawbacks are gone. It is greatly superior to the PzIV in armament, armour, off-road maneuverability, and speed. The lifetime of the engine is 700-1000 km.
- Less damage to the engines.
- There are no more breaches in side reduction gears.
- The controls and suspension demonstrate their qualities.""
CAMD RF 500-12462-93
700-1000 sounds like a very large number of kilometers. I guess we could take the 1st regiment's word for it that the T-34 isn't the shining star of the Eastern Front anymore, ignoring the fact that Guderian's "all drawbacks are gone" assertion seems to be contradicted by them.
However, just to be thorough, let's look at some numbers from the other side. The 6th Guards Tank Army discovered the following lifespans of their tanks:
"T-34: 2000-2500 km, 250-300 hours
IS/ISU-122: 1200-1800 km, 230-280 hours
M4A2: 2000-2500 km, 250-300 hours
SU-76: 1200-1800 km, 180-200 hours
Individual tanks made it as far as 3000 km, and their motors functioned for 300-350 hours."
These are the figures for all components. Even the weakest link of the Soviet and American tanks served twice as long as the Panther's engine. This is, of course, forgetting its final drive, the average lifespan of which was 150 kilometers.
Edit: this post at the Tank and AFV Blog contains more information on the Panther's technical woes throughout its service life.