Getting a new or used 6.5 TD? Stock engine or upgraded? Re-using all the same accessories or not? Any upgrades while you're in there (great time to add EGT/Boost gauges and applicable sensors/probes, possibly a mechanical wastegate, and whatever else tickles your fancy).
Are you having a dealer do it, local shop, etc?
I do believe AM General's book time on a straight engine swap is close to 20 hours. Not positive on that.
Depending on if you find a used takeout engine and get a good labor rate you might be down around $7,000 total. Order a stock engine from a dealer and have them do the work, I'd budget $15K.
1998 TD 4 Door Hard Top (HMC4) w/ 89K miles
Updated block and loaded with parts from Hummer Parts Guy and Blue Hummer
HummerDan's Hardcore Skid Guards, Fender Guards, & Locking Fuel Doors; Ham (VHF/UHF) & CB radios; Predator RapTorq4 PCM; mechanical wastegate; Cepek Adjustable Shocks; M101A3 trailer.
Stanadyne Performance Formula every fill-up and ScanGauge II full time.