I have had several mares over the years... some come into heat regularly and you definitely KNOW it when they do... some come into heat regularly and you would never guess it.
Just like some women have painful horrible periods and some have simple no problem periods... we (and they) are all different..
We just brought a new mare home, my older mare (11) has come into full blown let everyone know it heat twice in the last 5 weeks. The new mare (3 1/2) has yet to show being in heat.