You should spend around 90 minutes per week working on the online exercises and additional tasks. Weekly emails and notifications will tell you which unit you should be doing this week.
You can spend more time, of course - there's always something new to discover!
Don't work through all the exercises too quickly - plan in time for regular revision to beat the forgetting curve!
To stay motivated and practice different language skills, make sure your regular study routine is a mix of different activities within your training. To get an overview of the different training elements available, take a look here: https://portal.speexx.com/static/guidedtour/en/