Sizing a ROS 2 subscriber queue from publish rate and processing time

A ROS 2 node subscribes to a topic published at a steady $100\ \text{Hz}$. Its single-threaded callback takes $25\ \text{ms}$ of wall-clock time to process each message, and messages arrive uniformly. Using a KEEP_LAST history, what is the SMALLEST integer queue depth that would let the subscriber buffer all messages that pile up during the processing of one message, so that no message is dropped purely due to queue overflow in steady state? Assume the queue must hold the messages that arrive while one callback is executing, in addition to nothing else.