Once a year in the Triumph in late October / early November. It can have anywhere from 500 to 5,000 miles on the oil by that time. I have a small stockpile of K&N HP-2004 oil filters and usually fill with Formula Shell or similar conventional 20w-50. This car sees mostly leisurely drives and road trips... but it gets put through its paces if there's a decent run of twisties.
6,500 miles on the Honda since that's the usual maintenance minder interval for me. It really likes thin oil, but I've started to mix 0w-20 with 0w-30 to get something in the neighborhood of 10 cSt @ 100°C. I'll use up my stock of M1-110s and then take another look at what's out there.
The Tacoma gets PP 5w-30 and a PureOne oil filter every 5,000 miles. It was abused in its early life but seems to have escaped unscathed.