Welcome to FT. It is unclear where you are travelling from, so that will make a big difference. The short answer is that no airline will give you free upgrades to a premium economy cabin just for being a frequent traveller. Some FFP allow upgrades with miles or certificates, but these need to be earned first. Some airlines, like United, allow their higher status members to sit in economy plus, which is not premium economy but simply gives more legroom. Other airlines allow you to bid for an upgrade to a higher cabin. It really depends on where you are located and what airlines you fly. There is, however, no free lunch.