There are limits on how long the police can hold you before they must either charge you or release you. As a general rule, the police can detain you for up to 24 hours without charge.
A senior officer (at least the rank of superintendent) can extend this to 36 hours for a serious offence. Beyond that, the police must apply to a magistrates' court, which can authorise detention up to a maximum of 96 hours (four days) in total. Detention under the Terrorism Act 2000 follows separate, longer rules (covered by PACE Code H).
While you are held, your detention has to be reviewed regularly by a review officer — the first review within the first six hours, and then at least every nine hours after that — to check whether keeping you there is still justified. If it is not, you must be released.
The clock and the reasons for your continued detention must be recorded in the custody record, which you (or your solicitor) can ask to see.
This is a plain-English summary. For the exact rules, read the official PACE Code C and PACE 1984.