Learning languages in parallel is usually the right strategy.
The problem is that it not only takes hands-on time to build the mental infrastructure, but also calendar-time.
In general, if you learnt a certain Word X time ago, you have a good chance of forgetting it after X further time. There clearly is a distribution, but this general rule of thumb does generally hold. I know I could learn a few hundred words in a day to remember them the next day, if I really wanted to (using mnemonics). But they would fade within days, or weeks at the most.
The problem is that it not only takes hands-on time to build the mental infrastructure, but also calendar-time.
In general, if you learnt a certain Word X time ago, you have a good chance of forgetting it after X further time. There clearly is a distribution, but this general rule of thumb does generally hold. I know I could learn a few hundred words in a day to remember them the next day, if I really wanted to (using mnemonics). But they would fade within days, or weeks at the most.