First off, if you are a dedicated freshwater hobbiest then you will not find the demands of the saltwater world too much. If you are just a casual hobbiest that doesn't do much in the way of care for your fresh tanks, then yes, saltwater can be an eye opener. The general opinion of tank size is to start off with the LARGEST tank that you can afford as it is easier to maintain stable water conditions with more water volume. I would say that a conservative middle of the road estimate on time would be 15 minutes a day with a solid hour at least once a week. Most "slight" mistakes as you call them, do not kill fish, neglect will kill them faster. It is also much easier to make sure you are doing things the right way first, then fix "wrong" ways, later. There are alot of folks here that are eager to help you with getting started with the "right" ways.
One thing you will hear alot in this hobby is the saying "nothing good ever happens fast". Take your time and be very patient as you move along, using the waiting time to catch up on reading about things you have questions about and then ask lots of questions and you will be just fine.
On a final note, this hobby is not a cheap one and you need to be prepared to spend a good deal more than what you would for freshwater.