It depends on the device you are monitoring, for the majority of equipment at reasonable polling intervals the answer is almost none, but there are other devices where it is quite measurable and significant. Some examples would be large network switches with hundreds of interfaces, or load balancers with a high number of VIP's and nodes associated. It is impossible to estimate ahead of time so all you can do is try to benchmark against similarly configured test equipment or check with the vendors of the hardware to see if they have had complaints in the past about polling on their devices.
