How long does it take for SDP to be assessed?

Hi there, think it was the previous week I applied for Severe Disability Premium (had stopped my normal ESA payment of £275 for the 2 weeks total payment because of being granted Enhanced Living part of PIP) is there any idea of how long it takes approximately for SDP to be assessed at all?

Just wanted some idea if at all possible how long that might be before the backdated payment.

Thanks ever so much