I'm a firm believer that any training method should be tailored to suit you and your dog's personal needs and training objectives. If you look at the reason why a trainer recommends a certain thing, you can modify the exercise but still keep the intent. The intent of Fennell's "5 minute rule" of no interaction and affection when you return home, is to keep the dog from coming to believe that you are coming to pay respect to them the way a submissive dog/wolf would when returning to the pack. If your dog is crated, you can still ignore them for 5 minutes in the crate.
Personally, I ignore my crated dogs when I come home. However, it's not because I subscribe to any theories about wolf pack behavior as it relates to domestic dogs, but because I often have to put away my groceries, or help my 5 year old with his coat and shoes, or just because I'm tired and don't want to deal with my dogs right away. The other day they were in the crates for a good half hour after we got home before my husband asked "Hey, where are the dogs?" We had completely forgotten to let them out.
If you want to follow the exercise that Jan Fennell recommends, just ignore the dog in the crate when you get home. Rather than set a time frame, I would wait until the dog shows calm behavior, whether it is 3 minutes, 5 minutes or 10 minutes. If you're concerned that the dog has to relieve itself before then, then let the dog out of the crate and put it outside without acknowledging it or giving any affection. I think the intent of the exercise doesn't really hinge on some arbitrary time frame that someone else sets, it's more about the state of mind both you and the dog are in when you interact with each other.