Most applicants receive TSA PreCheck® approval in 3–5 days, though some applications can take up to 60 days. TSA states that after you finish enrollment, you typically get your Known Traveler Number (KTN) in that 3–5 day window; longer cases are uncommon but possible.
The application itself is short: the online pre-enrollment usually takes about five minutes, and the in-person visit (identity documents, fingerprints, photo, and payment) typically lasts about ten minutes. You enroll through an authorized provider (CLEAR, IDEMIA, or Telos), then add your KTN to airline reservations so PreCheck appears on your boarding pass.
Because some applications take longer, TSA encourages members to renew at least 60 days before their membership expires.
Source: TSA, “How long does it take to get approved?”; TSA PreCheck® FAQ.
Image: TSA officer checking ID; Transportation Security Administration, public domain, via Wikimedia Commons.