A lease agreement is valid, binding and enforceable upon execution by the parties. Unless the lease agreement provides a specific amount of time for one party or the other to return it after signing, there is no time limit, per se.
However, providing a tenant with a copy of the signed lease agreement as soon as possible is a good idea, so that the tenant is aware of his or her obligation and to make the tenant aware of any rules or regulations that may be associated with the property.