Recently, a reader asked me how long our bicycle tour across the country took and how we did the math to figure that out. Here is my response:
Our trip (4,250 miles) was about 3 months. We took 1 day off per week and 1 week off half-way. Started the evening of May 31 (7pm!) and ended somewhere around August 27th or so.
When I am planning future trips, I use an average of 60 miles/day + 1 extra day/week + a week off if applicable (trips greater than 2 months, or if you just want to visit a place like a National Park) + travel to/from the start/finish to figure out about how long the trip would take. Even if you THINK "oh, my average will be more than 60 miles/day!" I would still recommend using 60 (or even 55), especially if this is your first big trip. Using a number that is slightly lower than you expect to ride will give a buffer for mishaps, preventative weather, mechanicals, etc. Remember that 60 is an AVERAGE. One day riding 80 miles and another riding 40 averages out to 60.
So, for example-- The Sierra Cascades route is 2,362 miles. 2,362 divided by 60 is about 39 days, or a month and a week. Add 5 days for a day off per week, plus one day to get to the start, 4 days at the end (one to rest, one to sight-see/rest, one to pack, one to actually travel). That's 39 days + 10 days = 49 days. If I were taking time off work, I'd add more days to the start/finish for rest and packing.Something else that is useful in planning is checking to see how long the self-supported (not van supported!) Adventure Cycling group plans to take.
Because your trip is 6,000 miles and will take longer than a season to complete, I'd also be very aware of any passes you'll go over and whether or not they'll be open during the time of year you expect to be there. If you search around, you can find charts that list when passes opened and closed over the last 50 years and their average opening/closing date.
Hope that helps!