You are able to request a withdrawal at any time provided that the loans you are on are performing and have more than 1 month remaining on their term. To withdraw, simply go to 'My portfolio' and then click 'Sell Loans'. Enter the amount you wish to withdraw and click 'Sell'. It typically takes 1-3 days to sell your loan positions although this depends on market conditions as you will be replaced by incoming investors.
Once your loan parts are sold, you can click on 'Withdraw' on the dashboard to request the funds to be sent to your nominated bank account.