It has been a while since I dealt with NIC so I hope I am remembering this correctly - but it is not strictly correct to say the period of earning is not relevant for NIC. If you hold back a claim and get paid for say 2 months' work in one month, the danger is that your employer will give you only one month's worth of earnings threshold, and therefore you will pay too much NIC. For irregular payments, your employer should use the exact percentage method, and apply however many week's or month's worth of earnings threshold apply to the earnings period - you would need to check carefully to make sure they had done this.