Sure, it might have been nice if it was spelled out more explicitly. I look at it this way, we know that there's a milestone every 2 encounters, and rings were designed to gain power after a milestone, so the idea was definitely there that PCs would normally, or often, continue for at least 3-4 encounters. Most characters have AT LEAST 6 and often 9 HS. While its POSSIBLE to burn up to say 4 HS in a single encounter, we all know that this is unusual, and its probably safe to say that the average character burns 1-2 per encounter. Again this points in the direction of an adventuring day in the range length of about 4 encounters before you're stretching, and you hit your 2nd milestone at that point, putting the 5th encounter in reach.
Personally I don't have a specific pacing in this regard. I adjust pacing based on the engagement level of the players, as well as my feeling about what will feel most interesting and to provide variety. I've varied between 2 and 10 encounter days. On the average though, I aimed at a level break roughly every 2 to 3 adventuring 'days'. Sometimes if the day was really crazy it was one day, other times it might be 3 or even 4, but I like to keep the pace of advancement going.
I think one of the things that I dislike about a lot of D&D games is slow advancement. I want to see the story get on! Cut to the chase! This 5e game I'm a player in is doing that to me now. The mid levels are ENDLESS and filled with trash encounters that nobody really cares about. I mean, its RP and its OK, but frankly I've run 1000's of PCs in different games, probably literally. I don't need to experience all this trivia anymore. Get me to the part where I get to do something unique where I'll remember THIS PC, or to where I push it too far and it breaks and I get to go down in flames and try something new.
So, I run my game that way. This is a big reason I don't have XP anymore, and just give out a level whenever someone achieves a 'boon'. You do something special enough to advance your character in the world, it advances. Learn something new, go up a level, get a nice new cool item, go up a level, kill some stupid awesome monster, go up a level. It just works, and I can put paid to most of the issues of pacing at THAT level of things.
And yes, of course, we all long ago achieved mastery of the "when do you really get a long rest."