We don't have a limit and every year, one or two flights land without a clearance. It's not fully Kosher, but given that the crews involved were satisfied to do, I don't think there is a real problem. However, we have many more cases where landing clearances were given but the crew decide not to do so because they are not satisfied with what they saw ahead. Rigid, inflexible rules are best used for planning, not flight execution.
PM