What's the penalty for breaking a residential lease?
For example, in the Bay Area in Northern California.
By default, the tenant is liable for all rent until the end of the lease. E.g. if neither the tenant nor the landlord can find a suitable and credit-worthy replacement tenant (e.g. if the market has crashed), then the whole lease must still be accounted for by the original tenant, and the lost "rent" becomes "damages".
However, there is also a concept of damage mitigation, and California Civil Code 1951.2 explicitly defines that it's the landlord's duty to mitigate damages. This means that the landlord cannot simply sit still and collect the rent on an empty apartment. Because of this, some smaller landlords in California outright have a policy that you can cleanly break the lease by paying for 2 months of rent as a penalty.
(It appears that a good summary of various examples about landlord/tenant damage mitigation is available at UniformLaws.org.)
However, when it comes down specifically to the SF Bay Area with the ever increasing rents and the lowest residential vacancy rates in the nation, and also especially with the corporate landlords that already have sufficient resources in place to readily advertise and promote an abandoned unit, it can probably be argued that, in practicality, requesting more than one month of rent (in damages) as a penalty for breaking the lease is simply unreasonable.