Nasty how - attitude, cleanliness or....?
Yes, a cow that is in the routine of being milked at particular time will be impatient as her body will be demanding relief of the pressure of her milk supply. A cow's behavior at milking can vary a lot from cow to cow - some are very "nice milkers" while others will go their whole productive life being "that cow" (I've dealt with both). The frequency of milking will depend on how you manage your cow. If you have a cow that you want for just a little milk for your family you can leave the calf with her and the calf does all the work for you with regards to keeping her milked, making it less an issue of having a routine milking schedule, etc. If you don't have a calf on her/want all the milk she gives to be collected rather than suckled you generally milk a minimum of 2x a day - but *could* milk more frequently if you wanted to (a calf would suckle more than 2x a day) and just had nothing better to do all day long.