For tubular batteries, generally flat(those low height, old type looking) once from good brand should last 3-4 years, those tall tubular costly batteries should last 4 to 6 years. depending upon the uses and maintenance. Offcourse backup time will keep reducing with time and at the end of life, it will not be even half the backup of what you would originally get. After that you can exchange your old battery with new one and can get 1k to 1.5k discount easily. Tubular battery requires less maintenance, less frequent acid water topup (which you should always get from only original/authorized supplier.). And yes for your requirement 150 AH battery is more then enough, if power cut is not more then 6-7 hours. Btw you can always get a higher power inverter(i. e. 1000 or 1100 VA ones etc for a few hundred bucks more) with the same battery, this way your inverter will be easily take the load of other equipment when needed. Just for example, at one time I had desktop/tv/lights/fan running, but when washing machine was turned on, inverter started beeping with overload.
EDIT: When you get your invertor installed, make sure that all the power plugs, where your heavy power consuming appliance are used, is not connected with Invertor's power supply. For each house holds, their is separate wiring for powerplugs, so the installing person will do it easily, but sometime some kamchore people will directly connect it to mains.
