How long must a fast bowler rest between spells?

Under the ECB Recreational Cricket Safety Regulations:

• After completing a spell, a fast bowler cannot bowl again from either end until the same number of overs as their spell has been bowled from the same end. A 4-over spell needs 4 overs from that end before they can bowl again.
• A bowler can change ends without ending the spell, provided they bowl the next over the Laws allow from the other end. If not, the spell is over.
• Interruption under 40 minutes: a spell in progress can continue afterwards up to the maximum spell length. If it doesn't continue, the normal rest period applies.
• Interruption of 40 minutes or more (scheduled or not): the bowler can start a new spell straight away.
• Matches of 20 overs or fewer a side: if the competition limits each bowler to no more than the maximum spell for their age (for example 4 overs each), the rest period does not apply and those overs can be bowled at any time in the innings.

The daily maximum still applies, and once a player has bowled fast it includes any spin they bowl later that day.