i wear a heart rate monitor, i also ride converted rails to trails that cross a lot of roads, so i usually get a 1-3 min break waiting for traffic every 3 or 4 miles. i have a nice trail system near my house. i can ride 150 miles on paved trails and only have to ride the road maybe 15 of those miles
i stretch before riding then ride hard until my Heart-rate, HR, gets to 145 or so then let up and try to keep it between 130-150 ish. if i go over low to mid 160's i get nasty head aches about 2-3 hours after the ride. it also depends on how long i stay over and how often i get over 160 with the HR. also my toes and fingers get numb if i go over 160 with the HR.
when the HR gets close to 150 i go down a gear and drop my cadence a bit and usually the HR does the same. when the HR doesn't go down at a the usual pace i then stop for 5-10 min or so until the HR gets to about 110ish. then i go again. i usually take 1 to 2 breaks a ride depending on the wind and how hard i worked during the day.
my rides are usually between 20-60 miles depending if after work or weekends. after work, 2 X a week, it's 20-30 miles. weekends, 1 day, it's 30-60 depending on the weather, mood and time available to ride. i try to ride at least 100 miles every week.