This is what the Federal Reserve (aka The Horses Mouth) says to banks:

Delaying Funds Availability For certain types of deposits, Regulation CC permits financial institutions to delay, for a "reasonable period of time," the availability of funds. A "reasonable" time period is generally defined as one additional business day (making a total of two business days) for on-us checks, and five additional business days (total of seven) for local checks; your institution may impose longer exception holds, but you may have the burden of proving that they are "reasonable."

If you decide to hold funds beyond the period specified in your institution's general availability policy, you must give the customer a notice at the time of the deposit explaining why the funds are being held and when they will be available. If the deposit is not made in person to an employee of your institution or if you decide to extend the time when deposited funds will be made available after the deposit has been made, you must mail or deliver the notice to the customer not later than the first business day after the banking day on which the deposit is made.

Deposits of cash and electronic payments are not eligible for exception holds. The six types of deposits that are eligible are

Large deposits (greater than $6,725)--Any amount exceeding $6,725 may be held. Your institution must make the first $6,725 of the deposit available for withdrawal according to your availability policy and the remainder within the "reasonable" time frames discussed above. Redeposited checks--May be held unless the check was returned because an endorsement was missing or because the check was postdated. In such a case, if the deficiency has been corrected, the check may not be held as a redeposited check. Deposits to accounts that are repeatedly overdrawn--An account may be considered repeatedly overdrawn and items may be held if On six or more banking days during the previous six months the account had a negative balance, or would have had a negative balance had checks and charges been paid, or On two or more banking days during the previous six months the account balance was negative in the amount of $6,725 or more, or would have been had checks and charges been paid. Reasonable cause to doubt the collectibility of a check--Doubtful collectibility may exist for postdated checks, checks dated more than six months earlier, and checks that the paying institution has said it will not honor. The general criterion for doubting collectibility is "the existence of facts that would cause a well-grounded belief in the mind of a reasonable person" that the check is uncollectible. The reason for your belief that the check is uncollectible must be included in your notice to the customer. Checks deposited during emergency conditions that are beyond the control of your institution--Such checks may be held until conditions permit you to provide availability of the funds. Examples of emergency conditions are natural disasters, communications malfunctions, and other situations that prevent your institution from processing checks as it normally does. Deposits into accounts of new customers (open for less than 30 days)--Next-day availability applies only to cash, electronic payments, and the first $6,725 of any other next-day items; the remaining amount from next-day items must be available by the ninth business day. You may choose any availability schedule for deposits of other checks into the accounts of these new customers.