I switched when I finished a bag of grower close to estimated laying time. So for some hens they received layer just before laying and others in the flock ate the layer feed for a month or more before laying. You need to know the age at which your hens are likely to be laying; some breeds mature sooner than others: black sexlinks could be as early as 16 weeks and later maturing hens could be 6- 8 months old. That's why some people advise changing feed after the first egg shows up.
There is no rush to feed the layer. No reason to rush, at all. All feed has calcium, just not as much as layer feed. If you have a month, most likely, to go before laying really and truly kicks in, there's plenty of time finish a bag of grower, or just feed All Flock or Flock raiser.