This will be influenced by your age now and the age you'd like to retire at. In your twenties, you have time to build a bigger pension pot. This becomes progressively harder the older you get.
Annuities require roughly £1,000 for £1 a week of income. So, to get £250 a week for life, you'd need to invest around £250,000 in an annuity. Coupled with the full state pension, you'd get £415 each week.
Outgoings should be lower in retirement as hopefully you'll have paid your mortgage off, so to maintain your living standards you'll need about two-thirds of the income you earned while working.
In April 2015, the tax rules were changed to give people greater access to their pensions. This doesn't apply to the state pension or defined benefit pensions but does apply to defined contribution or money purchase pensions. Drawdown of pension income is taxed at marginal income tax rates rather than the previous rate of 55% for full withdrawals. The tax-free lump sum continues to be available.
There are six options available including, leaving the pension pot untouched, purchasing an annuity, getting an adjustable income (Flexi Access Drawdown), taking cash in chunks (Uncrystallised Funds Pension Lump Sum), cashing in the whole pot in one go and mixing any of the options.
Remember, you can keep working beyond 65, even part-time, and still receive income from pensions.