What is reasonable access for a father to see his child?

My son sees has his four year old daughter alternate weekends from friday teatime until sunday midday & has done so for over a year. He is self employed with sadly like many others no work at the moment but he pays maintenance & has a great relationship with his daughter. He & his ex were very young & incompatible but he tried to be responsible & make it work when she became pregnant on the pill, although he was not in love with her. He doted on the child but inevitably the relationship with his ex broke down (with no-one else involved) while the child was still under a year. Obviously his ex was very bitter & a long & heartbreaking tussle ensued with her ofter restricting &/or denying him access to the child on a last minute whim just to hurt him. He never gave up & trod on eggshells accepting any time he was allowed to avoid conflict & eventually his ex mellowed, gradually agreeing to allow him more & more time. They get on quite well now but he is never allowed his child on special occions so if anything falls on his weekend he always misses out. He would love to start having her longer from friday eve until the sunday evening on his weekend plus some holiday & some of xmas etc but he is wary of upsetting what he has & constantly fears she will deny him if he pushes for more time. What is cled as a reasonable amount of access for a father? He is a popular good clean living devoted young dad & his daughter adores being with him.