I rode to Arizona a few years back and rode 1000 miles each day for two days. My only problem was worrying about deer. I also rode the Iron Butt around Lake Ontario and Lake Michigan in under 24 hours. That's over 1000 miles and it wasmn't a problem. I think that everyone has their own individual limits and it's best to listen to what your head says.
I've thought about this for a while and realize that I don't have an answer in a number form, only my personal experience over the years.
The distance ridden relates to comfort, fitness and safety. Weather plays a huge part: extreme heat, wind and /or cold are very limiting, also fairing or "naked"? and if faired, how decent is the wind protection? If it's very good then you will suffer crossing the Mojave at 115 F. Road conditions and type are factors, a day in the twisty roads of mountains will tire the arms and the slower average speed will shorten the daily total, as will fighting a strong wind from a side or quarter crossing the plains.
Riding gear will help or hinder the above factors. Also it is the journey that counts, not distance traveled. There have been numerous times when I've turned around and re-ridden an amusing stretch of curves for the grin factor alone and frequently for photos. It is a holiday, right?
I always sort of plan the next day but leave the final vague end point to circumstance. I'm never disappointed or pressured this way and pressure to reach a certain point is dangerous on a motorcycle. Ride safe and enjoy.
Weather is a large consideration, what type of roads you’re travelling on, traffic what bike, solo or with friends… they all add into the equation. If the elements suck (wind, rain, dust, grasshoppers, etc) you can knock a chunk off the daily mileage. When I’m in a large group I like to keep it under 800 km in a day. By that time most people are showing the signs of fatigue. In a small group where you don’t have to wait as long at gas stations and/or stop for photos & bathroom breaks I can maintain several 1000 km days but don’t want to do too many in a row. Next week I’m off to the N.W. states to romp through some backroads and I’ve planned out for 600 km days... it’ll be leisurely with the option of taking side roads to lengthen things out if I feel that way. 600 km’s is a bit on the low end, but the multiple days add to the equation. I’ve done 12 and 14 hour days on various bikes (FZR’s, RC51 even my trusty KLR650) so ‘proving’ you can do it is not an issue. Now it’s about maximizing the enjoyment and you shouldn’t burn yourself out one day when there’s several to follow. Next week some of the best roads I’ll hit are going to be on Day 3. I’m planning on being well rested and ready to romp.