Hi, Elizabeth,
You'll find that most B&B owners are really flexible. Accent on "most"! Usually a designated arrival time is between 4 and 6 p.m. If you've prepaid, you should have no problem, but you should still give a courtesy call if you're arriving later. If you've made arrangements, but not paid ahead of time, it would be best to call by 4:00 to let the host know that you're on the way. That way, they won't give the room away if you don't arrive by 6:00.
We've gotten in as late as 10:30 p.m., after our hike in the Cuillins took twice as long as expected, and we had to take a taxi from Sligachan to Elgol to pick up our car. However, I kept in touch with our host to advise her of our situation. Yes, I did get mobile reception in Glen Sligachan!
Just for courtesy's sake, best not to arrive any later than 8:00 p.m. if you can help it. Most B&B owners have to get up early to get breakfast ready, and many like to have tea and biscuits for their guests in the evening.
Have a wonderful time in Scotland! If it's your first time driving on the left, just drive very carefully for the first couple of hours. After you get used to it, you'll have fun! We'll be over in July as well, so we may see you on the road.
Mike (auchterless)
p.s.: The supermarkets in Scotland have some amazing picnic items that you won't find in the U.S. Also, most have "meal deals," where you get a sandwich or wrap, drink, and dessert for around three pounds. Best to bring some plasticware for those yummy summer trifles and chocolate desserts (not included in the meal deal!). Also plastic knives for slicing some of the wonderful Scottish cheeses.