[Taneli]

My gut instinct would be to have them make enough rolls to get the combined margin of success to 30+ in you example case, and in case of a failure they would waste more time (and require more MoS to make up for it), and in the case of a critical failure they would face a mishap of some sort and then we would focus on that situation.

Or, you could just have them make one Survival roll and go with that, assuming that the jungle was "just for colour" and the meat of the adventure was the thing on the other side of the jungle. Heck, in a sufficiently high-powered / cinematic / skilled setting / party I could just state that "30 days of jungle later..." and maybe have them roll vs. Survival once in case I wanted to have a chance to introduce a minor complication, or call the attention to the awesome skills of the Survival experts in the party.

All that said, if anyone has any good ideas on the topic, I'd be interested in hearing you out too!