Sorry. I can't bring up your attachment. I get an error. The 4510 are BCD counters and so they cannot be used in place of the 4526 binary counters.
The clock frequency can be anything up to the max clock frequency of the counters (2.5 MHz for the 4026/4033; 5 MHz for the 4510/4526). If the clock is 256 Hz, it can take as long as 1 second for the conversion to take place; 2,560 Hz, 0.1 second; 25,600 Hz, then 0.01 second to convert, and so on. The faster the clock frequency the faster the conversion.
The convert pulse must be at least 200 ns wide. When the convert pulse goes high, the binary counters load and the BCD counters reset and are held that way until the pulse goes low. When the pulse goes low conversion begins at the next clock pulse and continues until finished.