Although there is no fixed term, you should consider stocks and shares ISAs a medium to long term investment of at least five years, and ideally not tie yourself to a particular end date. Both capital and income values may fall as well as rise and are not guaranteed. You may not get back the money you invested.
If you choose an index-tracking ISA which invests overseas, changes in exchange rates between currencies may cause the value of your investment and the level of income to rise or fall.
Legal & General has been licensed by FTSE International Limited to use the name of "FTSE ALL Share Index".