While you cannot set the scanner to not have a sleep mode at all, you can set the shutdown time to up to 255 minutes, which would be 4 hours and 15 minutes.
On page 1, scan Enter Setup, then RF Auto Shutdown Duration. On page 49, in Decimal Digits, scan the number of minutes (up to 255) that you want to elapse before automatically shutting down.
Preventing the scanner from shutting down will decrease the length of battery life between needing to be recharged, as it keeps the radio active.
This applies to the WWS 800 & WWS 850 scanner.