Take this information with a grain of salt until we can get an expert on NB IoT technology to answer this question, but the CE level should be set by the modem based on the measured downlink received signal power and increased automatically when the random access preamble retransmission limit set for the current CE level by the network has been reached.
How long it takes to reach this limit depends on the limit and the period between retries (waiting for the network’s random access response).

Deep indoor penetration is enabled by robust narrowband modulation techniques and redundancy (repetitions of transmissions). Dependent on the coverage situation of the device three different Coverage Extension levels (CE0, CE1, CE2), can be used. The higher the CE level the…

more repetitions of the transmitting signal higher the power consumption of the device lower the throughput higher the latency