When to Perform Ablution for One Who Has Incontinence
6-2-2002 | IslamWeb
Question:
I am a person who has to make ablution (Wudu) for each Salah. Should I make ablution after the time of the prayer or can I make it a few minutes before that?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
1) Scholars of Islam differed in opinion concerning renewing ablution for a person who suffers from enuresis (involuntary discharge of urine). Ahmad and ash-Shaafi'i and others believe that one who suffers from enuresis has to perform ablution after the beginning of the fixed time of the prayer. This opinion is based on the Hadeeth in which the Prophet said: “Al-Mustahaadhah (a woman bleeding outside the times of monthly menses or in her postpartum period), who has to abandon the prayer in her monthly period, should perform Ghusl at the end of her monthly period, observe fasting, perform the prayer and make ablution for every prayer.”
Ibn Qudamah 
said that such a purity is restricted to time, i.e. like Tayammum (dry ablution) for every prayer. In other words, if a person, in your condition, performs ablution before the Aathaan and this ablution is broken by any of nullifiers of ablution, you have to renew your ablution.
2) Imam Maalik and others believe that the one who suffers from enuresis (urinary incontinence) can perform ablution once and pray all the prayers as long as his/her ablution is not invalidated by one of the nullifiers of ablution other than enuresis.
This second group relied on the Prophetic Hadeeth in which the Prophet said to Faatimah bint Hubaysh 
when she complained of bleeding: “Just remain without performing the prayer according to the length of your normal menstruation, then perform Ghusl and perform your prayer.” They (i.e. this second group of scholars) said: 'The Prophet did not order her to perform ablution.”
Finally, we think that the first opinion (of ash-Shaafi'i and Ahmad) is safer than the second (that of Maalik). Thus, we advise you to perform ablution for every prayer just after the beginning of the fixed time of the prayer.
Allah knows best.