In DnD 4e,
characters tend to level every 8-10 or so encounters.
Depending on the exact Encounter Level of each fight (or noncombat encounter (eg Trap).
Faster if the encounters are above their Character level, slower if below,
Also faster if they are getting XP for completing a story/dungeon/quest,
and if you have any house rules like XP for good roleplaying.
I personally find calculating Encounter Level, and XP per encounter to be boring and pointless, and I just tell the party to level up when I feel it is appropriate.
Generally I like to do it every 4-6 encounters (which corresponds to every other session or so), but that is perhaps a little more rapid than most like. This is a house rule.
A Dragon (or any enemy) can be as tough as you make it to be.
Even if you don't want to modify its stats yourself,
there are a large variety of dragons in the monster manuals.
In the core monster manual , there is a Young Black Dragon, which is "Adventurers First Dragon". It is a Encounter Level 4 Monster on page 75.
I'm sure since I last looks there are lots more dragons in the new monster manuals.
Formerly known as Challenge Rating (or CR), this is your guide to how what level the party should be at when they attempt it. Or conversely, what level the party should be at before you throw it at them.
Mostly the book just say Level, but I'm going to call it encounter level here to distinguish it from the level of any character involved.
- a Level 4 encounter is balanced to be a solid fight for a party of 4ish characters of level 4, containing a Defender (eg Fighter), a Striker (Eg Rogue), a Controller (Eg Wizard) and a Leader (Eg Warlord).
- A Level 7 encounter would be a very tough fight for a party of Level 4 characters, but they should be able to get though, though they will have to use a lot of there daily powers.
- A Level 10 encounter will be a truly gruelling fight (for a lvl 4 party), that they should probably flee if they want to live .
- A Level 2 encounter is going to be be pretty easy (for a lvl 4 party), odds are no-one in the party will get hurt noticeably.
- A Level 1 encounter will be a push over (for a lvl 4 party), don't expect everyone in the party to even get to act. Eg the Controller might AOE all the enemies to death. Still might be worth throwing a CR party lvl -3 encounter sometimes eg a Single Goblin Scout spots you, if he gets away the war horde will know you are here.
In the monster manual or in a adventure module, each monster, or group of monsters has a Encounter Level listed for it (see below). Eg there is a Encounter Level for a gang of goblins with a goblin boss.
This is listed with the Role, for PC's the Black dragon is a Solo Lurker.
Solo means it is a valid encounter all on its own, Instead in this spot it might say Leader indicating it buffs others, or nothing, indicating it is designed to be part of a Encounter Group.
A solo monster is extra tough to take into account that it gets to make less actions -- it has one turn to act, while the party has one turn each to kill it.
(No Solo monsters will get destroyed almost instantly if the party can encircle them and all just beat down on them)
Lurker describes its its Roll -- like Leader or Controller, more details on the in the DMG.
There is a formula to guide you in calculating your own Encounter Level it is on page 56 of the DMG
(As I said I don't bother, but it is probably worth it when your starting out and can't judge the difficulty by eyeballing it and fudging/modifying stats on the fly).
If you just combine monsters into a encounter group, you can just add up the XP.
Most of this stuff is convered in detail around page 56 of the DMG
A Single Monster from the Monster Manual:
A Encounter Group:
Thanks @kviiri and @Tobold for the corrections (see comments).