Not long mate, i scrub it as it foams and then wash it off so its probably only on there for 10 mins max.

Another cheap but solution is to buy brick acid - its similar to wheel cleaning stuff but cheaper and available in all DIY type stores. It might be sold as industrial driveway cleaner. Its very strong, you need some sort of container that it wont melt and then leave it in to soak. Beware I've been told it can damage aluminum if left for too long, ive only used brick acid on cast iron blocks/heads before.