On my 900MHz PIII (512MB Ram) Eraser estimates 1480 minutes to do a 35 pass wipe using only the Free Space option on a 40Gb NTFS partition. It would take a lot longer if I had selected the cluster tip option.
You could try asking in that forum, though recently there seems to have been almost no traffic in there.
Having used DBAN, though not to do 35 passes (and I have an 80GB HDD) I would guess that it would take you some time. It dpends whether you wish to verify every pass, the last pass or no pass. verifying each pass can potentially double the time.
I'll hazard a guess and say that you should set aside at least a day to do what you wish to do, assuming you verify the last pass (it is recommended to verify at least one pass). There have been stories of 36 hours+, thogh obvioulsy it all depnds on your computer.
Try that forum and see if you get any response.
Incidentally, I don't believe the 35 pass Gutmann method to be the most appropriate pass, it is based on a paper he wrote in 1996, and computers and how they operate (store data) has come a long way since then. When I use DBAn I use the PRNG (random pass) option, using the following as a guideline
4 pass PRNG - moderately secure wipe
8 pass PRNG - highly secure wipe
12 pass PRNG - just to be on the safe side!
Note: for the PRNG I get "pass" and "rounds" mixed up, it's either 4 rounds of one pass, or 4 passes of 1 round. I'd have to load DBAN to jog my memory.
I use the verify last pass option, usually do the 8 pass / rounds and it takes about 4 to 5 hours. That wipe is probably as secure as it will ever need to be - if I needed a more secure wipe, destruction of the disks would probably be more appropriate, and I never have anything on the computer THAT sensitive, just banking and other personal details...