Every tax year (which runs from 6th April – 5th April), anyone over the age of 16 receives an ISA allowance, which they can use within that tax year. If you do not invest in an ISA in that tax year, the allowance is lost.
You can invest up to £7,000 into a Maxi equity ISA, or if you prefer Mini ISAs, up to £3,000 into a Mini cash ISA, and up to £4,000 in a Mini equity ISA (previously the limit was £3,000 for Mini equity ISAs).
You may not have both a Maxi and Mini ISA in the same tax year, but you are allowed to take out both a Mini Cash and Mini equity ISA. Also, once even a single pound goes into any type of ISA in a tax year, you then have to continue with that particular ISA, regardless of whether you continue to pay into it or not, so think carefully before investing.
You can invest as a one-off lump sum, set up a monthly plan, or pay-in whenever you like during the tax year, as long as you do not go over the relevant limit.