Whats the longest time you can keep a sheltand sheepdog in its crate?

im thinking about getting a shetland sheepdog but when im at school no one will be able to look after it, and i dont want to keep it in its crate the whole time whilst im at school. but i dont want the dog to start chewing everything up round the house if i leave it to walk round on it own. What should i do? and if i did crate it, how long would the shetland sheepdog be able to stay in it for without getting bored and upset?