Most houses are expected to settle over time, but foundation sinking of more than a few inches or uneven sinking of more than an inch for every 20 feet is cause for concern. Any significant or uneven sinking should be assessed by a professional to determine the cause and perform any necessary repairs. 

Concrete leveling can last anywhere from five to 10 years or more, and in some cases, it can be a permanent solution. The longevity depends on the method used and whether the underlying cause of the sinking is addressed. For example, mudjacking typically lasts five to 10 years, while polyjacking can last for 20 years or more. It's best to consult a professional for an accurate timeline based on your specific situation and any factors that could cause the issue to recur.

A good slump height really depends on the project, but there is a use for nearly all slump levels. For example, low slumps are best in projects that require structure, such as curbs and roads. High slumps often require additional reinforcement, but they flow more freely into hard-to-reach spaces.

The average cost for mudjacking is around $1,200, though prices can range from $300 to $3,400. On a per-square-foot basis, you can expect to pay between $3 and $8, with an average of about $5 per square foot. The final cost depends on several factors, including the square footage of the concrete, the type of slab, its accessibility, whether it is load-bearing, and the type of material used to lift it. Generally, lifting a driveway, small patio, or sidewalk is less expensive than lifting a slab foundation or garage floor.