My understanding is that early presentation to hospital with ischaemic stroke does not improve survival [1]. The reason for early presentation is that if thrombolysis with clot busting drugs is administered early then long term neurological outcome is better. Survival alone is not improved however.
As discussed in another comment, the earlier the better. The idea is for as little time as possible to have elapsed between blocking off the blood supply and opening it up again. The reason for this is that when part of the brain becomes ischaemic, a central core dies instantly but around this there is a halo or penumbra that is not dead but at risk. This penumbra of tissue dies off over time, and can be saved if the blood supply is reconstituted. The penumbra can be visualised using quite clever MRI [2]. Having said that, trials that have tried implementing this kind of imaging to guide thrombolysis have not been positive, as compared to just thrombolysing everyone.
As discussed in another comment, the earlier the better. The idea is for as little time as possible to have elapsed between blocking off the blood supply and opening it up again. The reason for this is that when part of the brain becomes ischaemic, a central core dies instantly but around this there is a halo or penumbra that is not dead but at risk. This penumbra of tissue dies off over time, and can be saved if the blood supply is reconstituted. The penumbra can be visualised using quite clever MRI [2]. Having said that, trials that have tried implementing this kind of imaging to guide thrombolysis have not been positive, as compared to just thrombolysing everyone.
[1] http://www.ncbi.nlm.nih.gov/pubmed/23791822 [2] http://en.wikipedia.org/wiki/Perfusion_scanning#MR_Perfusion