You can cash in your investments and withdraw your money whenever you like. There is no minimum term and no charge for taking your your money out.
To withdraw your money, simply log into your account, select the 'manage money' tab, click on the 'withdraw' button and type in how much you want to withdraw.
We will sort out the rest, including filling in any forms you need to sign and posting them to you with a stamped addressed return envelope.
However, bear in mind that investments will go up and down so investing is generally for the medium / longer term.