A worked example: how long does a 5+3 game actually take?
Take a standard blitz pairing, 5+3: five minutes of base time plus a 3-second increment added after each move. Assume a 40-move game (40 moves per side — that's our assumption; shorter games use less time). Your maximum clock is 300 seconds plus 3 × 40 = 420 seconds, or 7 minutes per player. Since both players face the same math, the game physically cannot run longer than about 14 minutes. Run the same arithmetic on 3+2: 180 + 80 = 260 seconds, roughly 4 minutes 20 seconds per side, under 9 minutes for the full game.
Now compare 15+10, a common rapid setting: 900 + 400 = 1,300 seconds, about 21 minutes 40 seconds per side, so a 40-move game takes a minimum of roughly 43 minutes — and closer to an hour if it stretches to 60 moves. That's the practical payoff of the notation: "quick" time controls aren't always quick, and a 15+10 game doesn't fit a 20-minute lunch break no matter how fast you play. Plug in your own base, increment, and expected move count before committing, and budget for the worst case, since clock math sets the ceiling, not the estimate.
Common mistakes and edge cases
Don't confuse increment with delay. The notation 5+3 (Fischer increment) adds 3 seconds to your clock after every move, which you can bank; 5|3 (Bronstein or simple delay) makes the clock wait 3 seconds before counting down, with nothing banked for moving quickly. Increment also posts after your move completes, so with half a second left you can still flag in the middle of a move. Online, note that servers apply lag compensation — the sub-second figure you see on your clock is smoothed and may not match the raw server clock, so don't count on winning a dead heat.
Classification boundaries are conventions, not laws. FIDE defines blitz as under 10 minutes per player and rapid from 10 to 60 minutes, which makes 5+3 blitz and 10+0 rapid — but USCF and individual websites draw the lines slightly differently, so the same setting can sit in different pools on different sites. Flagging rules have their own edge cases: under FIDE rules, if your opponent's flag falls but no sequence of legal moves could ever checkmate you (say, a bare king), the game is a draw, not a loss — and sites enforce versions of this inconsistently, so check the rules where you play. Finally, remember increment changes endgame play: in 5+0, K+R versus bare king becomes a speed race the attacker wins by flagging you, while the same position at 5+3 is trivially winning and calm.