You should have both in any case. Whether by a service or by a lawyer, it depends on your project.
Again, it all depends on your business/project, which I have no information about.
There are some clear red flags for services at which moment they should seek the help of lawyer in any case:
so much for the "necessary" part of your question. When it comes to the time spent on it I think it's fair to say that in the beginning you'll get away with little time spent on it and the bigger you get, the more time you need to invest. It's just natural: you'll deal with more data, by more people and your business model might be more complex and spill over into the terms side of things.
However it's definitely good to think about it from the beginning and in doubt not collect data if you don't need it (also definitely encouraged by GDPR for instance).
Support for all your back office needs.