there's about 3 or so signage, counting from the base of the hill... but dun worry, it's a one way trek up the hill, so u won't get lost... and besides, there will be other tourists going the place, so all just crowd together and go together

you won't get lost..
In normal summer season, and if you never carry heavy load, I guess with reasonable fitness, the trek can be completed within 30mins. However, going in winter season (and if u have a heavy load), u will be slowed down... the road (which is covered with snow/ice) is really very slippery... Be very very very careful when u walk... take small steps aka walk like the penguins.. if u are able to buy the snow spikes attachments to wrap around ur shoes, it will be perfect (I dunno where to find the snow spike attachments in Tokyo, Osaka, Kyoto, Nagano... those convenience shops/department stores/Tokyo hands/takashimaya etc. doesn't sell them...) I know u can buy in Singapore northface for $49 - it may not be worthwhile for 1 trip... so just walk carefully..
yes, the journey to-and-fro ate up 90% of the day - I departed at abt 7am from Shinjuku, and reach back Tokyo in evening at abt 7-8pm (and I only managed close to just 2 hours of shooting!), which is why I think departing at 6am is the most ideal.. And don't forget, u have to wait for the local train, u have to wait for ur shinkansen (frequency abt 2 hikari class shinkansen per hour for u if u use japan rail pass, so u can expect to wait abt 20-30mins for the shinkansen), and for the limited express train from nagano to yudanaka (the waiting time is again abt 15-20mins), and for the bus from yudanaka to the snow monkey park (personally I waited abt 18mins)... So guess this will give u a clearer picture why it takes so long.
I dunno how much load (camera eqpt + winter clothings) u will be carrying, but if u have ipad/nexus 7/galaxy tab, it would be good to bring with u to help u kill the train journey time... for my case, I only played with my iphone, as my bag was too heavy to bring ipads.. (frankly, after this recent japan trip, I am rather "scared" of the long distance shinkansen and train journeys.... they are simply long and boring and I felt like I am locked in a moving prison..)
All in all, the highlight of the entire day trip to the monkey park, is really just the monkey and the snow covered surroundings.. all the rest didn't excite me at all, but rather bore me down tremendously...