While it is almost impossible to figure out the exact time in advance, there are a couple of factors we can take into consideration while trying to find out approximate ride time. Most of us use Google Maps to get an estimate before we head out. We will continue using those as our base time for all calculations. Few factors to consider over and above what google/apple maps show :
1. Number Of Riders
The biggest one for me is the number of riders/motorcycles. More the riders, more the halts, more the issues and more of everything that would need us to calculate additional time. Below is an approximate addition to the time maps shows you, depending on the number of riders :
Riders = 1 = Google/Apple or any other map should show you the approximate time
Riders = 2 = Maps time + 10%
Riders = 3 = Maps time + 15%
Riders = 4 = Maps time + 20% and so on
2. Number Of Days
The above estimate assumes a single day trip but lets say you are going for a multi day trip then you need to add atleast 10% more. This additional time will be consumed in the form of everyone not waking up at the same time, others getting tired sooner, smoker halts, breakdown etc.
3. Google Time Hours
Hours shown on map are strictly riding hours. They do not take into consideration halts, rest area etc. Bikers tend to take more halts than a car. I would typically consider a max 8 map hours of riding per day, especially on a multi day trip. For every 2 map hours, you must add another 25% (30 mins) as rest time.
4. Weather
Another consideration that most of us ignore is the weather. If its too hot or too cold or rain, it will definitely add up to the hours. You are going to need a few more halts than normal. For Eg if you are in AZ at 110 or in Utah at 35, either of these are going to force you to take more stops or then go slower
5. Direction | Timezones
If you travelling across states, keep in mind timezone changes which also result in an early or delayed sunset/sunrise. While this has no impact to the number of hours you are on the road, it could be a significant factor in calculating the distance you want to travel in a day. For Eg if you do not like riding at night and you are travelling West to East across timezones, not only will you lose an hour but it will also get darker sooner than you expect
Example calculation :
If we are 3 riders and the distance we want to cover shows a total of 20 map hours, then this is how I would calculate :
20 map hours + 3 hours (15% for 3 riders) + 2 hours (10% multi day trip) + 5 hours (25% rest time) = 30 hours
Assuming we are willing to be on the road for 10 hours a day, this trip can be completed in 3 days