How long will the batteries last in a 1kVA UPS before they need changing?

How long UPS batteries will last is dependent upon several factors including their design life, type, the number of charge/discharge cycles they are put through and the ambient temperature of the room they are working within.
Lead acid VRLA UPS batteries can be of the 5year or 10year design life type. These will require replacement within 3-4 or 7-8 years respectively. In terms of room ambient, the recommendation for lead acid UPS batteries is 20-25°C. For each 1degree rise above 30°C the design life halves. Lithium-ion UPS batteries have a longer design life from 10years or longer but have a higher cost than lead acid types.
When you purchase a 1kVA UPS, the product warranty will generally include the batteries for the warranty period. Some UPS manufacturers limit the warranty to that provided by their battery supplier, and this may be on a pro-rata basis i.e. 100% warranty cover in year 1, 80% in year so and so on. Some UPS companies will offer extended battery warranties and battery cover including testing and replacement as part of their UPS maintenance contracts.