We sought to explore the therapeutic effect of botulinum toxin (BT) therapy by analysing the time between the BT application and the onset of its decrease (treatment duration, TD), the inter-injection interval (II), and the excess time (ET, ET = II-TD). For this we studied 59 patients (37 females, 22 males, age 52.6 ± 10.9 years) with cervical dystonia (CD, Tsui score 9.0 ± 4.1) and BT therapy with Botox(®) and/or Xeomin(®) sequentially. Altogether 1,289 treatment cycles were evaluated. On average 21.8 ± 14.0 (4-66) treatment cycles were recorded for each patient. TD was 11.8 ± 2.7 weeks (7.8 ± 1.4 to 21.0 ± 3.9 weeks), II 15.4 ± 3.4 weeks (11.3 ± 1.3 to 27.8 ± 11.6 weeks) and ET 3.5 ± 2.4 weeks (23% of II). TD and II were stable throughout the treatment course. In 36% of the patients we found TD ≤10 weeks, in 83% TD ≤12 weeks. In 17% of the patients we saw treatment delays due to appointment difficulties, due to the patient's attempts to explore TD or his actual CD severity, from fear of adverse effects or due to psychiatric comorbidity. 19% of the patients showed prolonged treatment effects probably due to CD fluctuations. 0.38% of the injection series produced singular unexplained therapy failure (SUTF). Antibody-induced therapy failure did not occur. TD and II are stable on long-term monitoring. SUTF, treatment delays, and CD fluctuations can occur. 23% of the time patients are treated suboptimally. Our data suggest to reduce II. If II is to be reduced to ≤12 weeks, use of low antigenicity BT drugs might be useful.