After 30 days from your account opening, you will receive £75 cash in your account that you can instantly withdraw. This offer is extremely profitable because you are receiving cash and not dealing with any bonuses or free bets. If you stick to 95% matches from the Oddsmatcher then you will make a guaranteed £60 profit at least from this promotion.
You can try to request the £75 sooner than the 30 days if you have 30 settled bets of £10+, however the terms state you will have to wait 30 days so you might not be credited instantly.
Please note also 188Bet only allow you to withdraw in whole numbers, so you may wish to choose odds for your free bet that mean you'll end up with a whole number (or close to it e.g. £24.10 rather than £24.99 winnings!)