There is no brand-wide (let alone company wide) norm for check in or check out. Each hotel has their own times and rules. When hotels offer early check in, they will specify from what time it comes just like paid late check outs. It's frequently offered in places where many flights are likely to arrive early in the morning and hotels know that many passengers will ask for an early check in (e.g. Australia and New Zealand) which can thus be monetised.