Theoretically a 450W load translates to 38A of current at 12V, assuming there are no losses. Now if you are having 2.25 Hrs backup, this means your battery is still 2.25 x 38 = 85AH. But there are many factors which you must consider before expecting longer backup times.
1) Local made UPS efficiency is low, around 60-70% max.
2) A battery claimed 200AH by model number (e.g. NS-200) is not actually 200AH, it is at least 20-30% less.
3) Even the actual AH of the battery is only available under certain conditions, which are hard to meet while using the battery in a UPS.
4) AC loads should always be calculated in VA, not Watts. The term VA takes into account the power factor as well, and VA rating is always greater than Watts in most of the domestic loads. This means if your load is 450W, it looks to UPS as 600-700VA, which further explains a shorter backup time.
5) Your battery is already about 5 months old, the AH capacity reduces quickly when UPS battery encounters many charge and discharge cycles.
6) The battery already suffered an irreversible damage due to dryness of cells. You don't know how long it has been dry.