Translating "should" to express what is probable or expected
Generally, you can use 应该. However, as Stan says in comments, it does carry the connotation that there is some reason why it "should" be. So, for example, you could say a sentence like:

[要是他六点离开了，]他应该七点到。
[If he has left at six, ] he should be here at seven.


If you're just throwing out an estimate, it would be better to use an "I think..." construction. This carries less of a tone of uncertainty than it does in English:

我认为我周五会把这个任务办好。
I think I'll have it done by/on friday.