This says the crystallisation happens rapidly
"Following subcutaneous injection, insulin glargine is rapidly metabolized into its two main active metabolites: M1 (GlyA21) and M2 (GlyA21, des-ThrB30) [
97], with little or no glargine molecule being detected in the systemic circulation. The M1 metabolite accounts for approximately 90 % of the available daily plasma insulin [
97], and its release from the poorly soluble parent compound is the primary mechanism, resulting in the pharmacokinetic characteristics and consequent pharmacodynamic effect with the long-acting time–action profile observed with insulin glargine treatment [
98–
100]. Steady state is attained within a few days [
101]. Importantly, adverse events, injection-site reactions, and antibody formation with insulin glargine were found to be comparable with NPH [
75]."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4045187/
Logically it would have to be a very rapid process, because otherwise other things would start to happen. I would assume it's pretty fast.