First off, this is for those who want to play an expansion, not just level through it in a hurry.
Some terminology that can be a bit confusing: Chromie Time is where you select an older expansion to play in. That will put you in a separate ‘timeframe’ from the ones playing ‘current time’, you can’t see them or interact with them, only with players who are in CT too. Exception being in the capitol, and the AH which is the same for all.
Timewalking is a special, week-long event once a month, from wednesday to wednesday. During TW week, you get extra reputation from doing quests in a particular expansion, and you can queue up for that expansion’s heroic dungeons. They come in sequence from TBC to Legion, (maybe BfA now?). TW is available for all players that have the current-time max level for that expansion. That’ll be 35 up to and including MoP, 40 for WoD, 45 for Legion. TW is availbale across expansions and whether you’re in CT or not.
You will hit max level in a good, long yawn’s time these days, so you will need to lock your xp at some point to stay in the content. Currently, you’ll get kicked out of chromie time at level 70 and that’s final. In other words, lock your xp at 69 if you wish to stay. At 69 or below, you can hop in and out of CT to your heart’s content.
However… the launch of TWW has seen a return of a very unwelcome bug. Once you hit level 61, the dungeon finder does some odd filtering, and narrow your options down to just the starter dungeons, those you see at level 10. For some expansions, like MoP, that means you’ll have literally one dungeon to queue for. Thus, if you want to hit all dungeons along the way, lock at 60 at the latest. They might fix it at some point, but it was in place for 1½ year doing DF, so… yeah.
TWW introduced some sort of level scaling voodoo as well, maybe to ease leveling to 70, who knows? at any rate, from 59 and into the early 60s, MOB health doesn’t scale, it stays the same. If you don’t find it overly engaging to run around and steamroll everything, lock at 59.
As a curiosity, at level 35, (only at 35), you can queue for Hour of Twilight dungeons and Cataclysm heroics. It’s most likely another bug, and people race past level 35 anyway, so you can queue, but I haven’t tried actually getting in yet 
Level 35 is a good level if you want to work alchemy along the way. At this point you can actually get a little benefit from what you make. Same goes for enchanting, which can be applied to items up to iLvl 320 now, but the bonueses are completely meaningless at higher levels. That scales to 45 in WoD and 50 from Legion onwards.
Also, at 35, you have most of your core mechanics in place, and is strong enough that you can drop out of CT and solo bosses and dungeons without the nuke effect from being too high level. Some boss mechanics makes it a no-go, but those are fairly few. You can also queue for Timewalking dungeons when those are up.
Dungeon running is completely dominated by the get-to-70-ASAP mentality, so it’s going to be a race to the end-boss and the satchel. If you want to do full clears, or just take it easy, use the group finder instead and see if you can find some like-minded people there. Or solo, as mentioned above.
It’s worth noting, that you don’t need to be in the CT expansion you’re playing. Put differently, you can key in to TBC, for example, but play the quests in Pandaria. No problem. The only thing affected by the expansion you choose is the dungeon finder and that it triggers the relevant introduction quests, if any. Some expansions are more popular to level in than others. As a result, you might find yourself sitting forever in the queue without anything happening, so just remember that you can switch to a different set of dungeons by changing your current expansion.
It’s a good idea to go pick Legion right away, accept the intro quest, and then you can switch immediately to whatever other expansion you want to play, if not Legion. With the intro mapped, you go talk to a guy where you can choose to skip the intro questline if you’ve already done it. Otherwise, just do it. Both ways will land you in Dalaran, where you’ll get a second hearthstone. This little gem will enable you to whoosh yourself to Dalaran every half hour, and from there use the portals to your capitol cities. Cuts a lot of travel time, if you’re not a mage.