currently i only have a header and exhaust but i was just curious if you need to go in for oil changes more frequently with modded cars?

Heat and contamination by dust/dirt are the two main concerns (barring detergent effects by chemical additives that may break down the lubricating abilities of the oil). Heat breaks down oil, dirt as you'd guess acts as an abrasive to the cylinder walls, bearings, etc.

Unless youre running a turbo/supercharger (added heat) or a poorly made airfilter (dirt) I wouldnt worry about it.

Most people drive under the "severe service" schedule as shown by Scion. Do it every 3K miles or 3 months, regardless of mods.