It is not always the same for cows to resume to it’s normal estrus cycle after giving birth. Even it can vary for the same breed depending on several factors. 

Nutritional Status:  If cows are in good dietary conditions, normally time is less to regain the normal condition. As the body is stressed, additional energy and minerals are needed, and there are always some imbalances. Some breeds can regain fast on the other hand some take more time. Farm facilities and food supplies can also impact on these. 

Stress level: Not all cows go through the same stress level during the gestational period till giving birth. As stress level varies, the postpartum estrus cycle also varies. Retained placenta, uterine infections, or metabolic disorders can delay the onset of postpartum estrus. Moreover, stress factors like poor housing, rough handling, or overcrowding can also impact the timing of the cow’s return to heat.

Difference of Breed: Based on the metabolism and energy demand, different breeds need different times to return to heat. Normally dairy breeds take less time than beef breeds to get into heat. 

Management: Due to suppression of reproductive hormones caused by cows suckling their calves may experience a delay in the estrus cycle. Early weaning can shorten the reproductive period. 

There can be many more factors like first calf heifers can take longer time than multiparous cows.