It actually takes at least 5 days for the depot to become fully established when you're just starting out on Lantus. From the [b]Tight Regulation Protocol sticky[/b]:
"General" Guidelines:
Hold the initial starting dose for 5 - 7 days (10 - 14 consecutive cycles) unless the numbers tell you otherwise. Kitties experiencing high flat curves or prone to ketones may want to increase the starting dose after 3 days (6 consecutive cycles).
Each subsequent dose is held for a minimum of 3 days (6 consecutive cycles) unless kitty earns a reduction (See: Reducing the dose...).
Adjustments to dose are based on nadirs with only some consideration given to preshot numbers.