PrE has no way to "span" multiple DVD-Videos. It also balks at really long Timelines. My advice would be to assemble your footage into Projects, each one for one disc, so you can keep the quality high.
If you already have everything in one Timeline, I'd then trim each to equal one disc, do a Save_As (Conference_Disk_01, _02, etc.) for each trimmed Project. Do the navigation for each disc, and then burn. Personally, I'd include Titles at the beginning and the end, to alert the user that they have, say Disc 1 of 6, and also "Insert Disc 2 now." That sort of thing.
PrPro has a few more capabilities, such as separate Sequences (think mini-Projects inside of a master Project), but I do the multiple disc sets the same way. Each Sequence (or groups of Sequences) is a separate disc, and is Exported for Import into Encore (my authoring program) and each disc is burned at the highest quality that will fit. Just finished a 17 disc Project, and it was handled the same way. Each Sequence was titled with the disc # and the placement on that disc, so I knew what went where. Had about 80 separate Sequences, but it was easy to know which ones went into which DVD/Encore Project. I also had the Disc X of Y on both the front of each disc, plus on the case for it. That way, the user knew where they were in the order, and could get the disc back in that order. You can do the same thing in PrE, but will be using Projects for each disc. A bit of planning will go a very long way in the production.
Good luck, and let us know if you have any more questions. BTW - for acceptable quality, I'd set a limit of 2 hours for any duration, with maybe 150 mins. as the absolute max. I've "stuffed" 3 hours on a DVD-5, but it is NOT a pretty sight. Can you go with DVD-9's?