For Summoner's Rift (5v5)
Baron Nashor - 15:00 initial, 7:00 respawn after death
Dragon - 2:30 initial, 6:00 respawn after death
Lizard/Golem - 1:55 initial, 5:00 respawn after entire camp death
Wolf camp - 1:40 initial, 1:00 respawn after entire camp death
Wraith camp - 1:40 initial, 0:50 respawn after entire camp death
Small golem camp - 1:40 initial, 1:00 respawn after entire camp death
For Twisted Treeline (3v3)
Ebonmaw (Dragon) Buff - 4:40 initial, 5:00 respawn
Grez (Lizard) Buff - 2:10 initial, 4:00 respawn
Ghast/Rabid Wolf Buff - 1:55 initial, 3:00 respawn after entire camp death
All small camps - 1:40 initial, 1:15 respawn after entire camp death
Not sure if these values are 100% accurate for Twisted Treeline after the recent jungle remake. If anyone has specific info, feel free to comment/edit it in.
The values listed for the Ghast/Rabid Wolf and Lizard/Golem initial spawn are 1:55 officially in the wiki, however this appears to vary by about 1s and in practice they're actually a bit closer to 1:54 on the in-game clock
As indicated, a camp of neutrals will not respawn if even one mob is left alive in that particular camp. The timer to respawn will not start until the last mob is killed. For example, if I kill the Blue Golem at exactly 3:00 but leave the small casters alive, the Blue Golem will not respawn at 8:00, but rather, 5:00 after whenever those two casters are killed.
Also, unlike HoN / DotA, neutral mobs will still respawn if there is something obstructing their spawn point (e.g. a ward, an object like Shaco's JitB or a Nidalee trap, a player, etc.)