Almost instantly (assuming idle or near idle speeds at shutdown). Engines don't like doing work with no fuel and just the act of turning the compressors as it spools down will ensure that any ability of the fan to force significant quantities of air through the engine will be gone within seconds.
Don't forget that the fan is essentially a so.lid disc without much momentum due to its light weight. You're then asking it to do work as it decelerates anyway
*caveat - I'm not prepared to test this theory personally.