Is it typical for a Contractor to request 20% or more upfront?

[Jon Mbu]

Our General Contractor is requesting a 28% downpayment upon signing the contract for rehab work, prior to any work being started and he claims that he has been doing this for decades and never had issues with that being a part of the contract.  That seems rather high.  What amount (if any) should typically be required from a General Contractor upfront?  If you are a rehabber, what was the most you had to pay upfront when a GC requires a deposit?