The air bar is immediately underneath the happiness bar beside the character status window for the associated blockhead.
Normally not displayed, the air bar appears only when a blockhead is underwater or in space. It has a round, blue "bubble" icon beside it.
When underwater, it takes about 30 seconds for a blockhead's air bar to run out. Once an air bar runs out, a blockhead will start to lose health. It takes about 20 more seconds past the air bar's expiration for the blockhead's health bar to reach empty. It will take another twenty seconds before the death bar empties and the blockhead to die, meaning it takes roughly 70 seconds or 1 minute and 10 seconds, for a blockhead to completely drown.
Being in space has similar results.
While decreasing, the air bar will change from blue to red beginning at the far end from the bubble icon and progressing toward it.
The air bar will increase once the associated blockhead's head is back in breathable air. The progression of red will quickly reverse (2 seconds) until the bar is completely blue, at which point the air bar will vanish.