When you purchase a bat it really should be hand selected and weighted to suit your requirements. I suggest going too your local sports store and have a look around and find a weight and pick-up that suits you. Better yet, select a few bats you like the look of on ebay and take a day trip too one of the larger specialist cricket stores (Greg Chappell, Kingsgrove... etc). You'll then be able to get a good feel for the bat.
Your cricket bat is a "Tool of the Trade"; it's raw materials are grown and not manufactured, and therefore, it's expected that the condition of the bat will deteriorate during its usage. The performance of the bat should be unaffected, and as I have found often increases with age if correctly care for it correctly. The playing life of your bat relates directly to the preparation, maintenance and care of the bat throughout its life, the following are notes to assist the explanation of situations which may occur within the lifespan of any cricket bat.
1. CRICKET BAT WILLOW - English Willow (Salix Caerulea or Alba Var) is, by nature, a soft fibrous reed. Performance and durability are enhanced by drying and pressing. In normal usage, Willow will become bruised, scarred or dented simply due to the nature of the game (the impact of the ball). The performance of the bat will not be affected. Such marking can occur at any time during the lifespan of the bat.
2. KASHMIR WILLOW - Willow grown outside of England is, by nature, much harder than English Willow. Therefore, bats made of this type of Willow will not have a comparable level of performance with English Willow bats.
3. LIFESPAN OF A CRICKET BAT - There can be no definitive length of time that a cricket bat will last. The condition of a bat deteriorates with use. The length of time before the player needs to replace their bat will depend upon:
a) The amount of use.
b) The weight of the bat (heavier bats tend to be more durable than lighter ones).
c) The care with which the bat is treated.
4. IN CASE OF DAMAGE - Immediate action should be taken. This normally necessitates withdrawal from play whilst repairs are carried out.
5. SURFACE CRACKS - Surface cracks do not harm the bat's performance and all bats will eventually have some surface cracking. To maintain these:
a) Glue any parts of the blade where Willow is 'lifting' and clamp with string or woodworking clamp. Use any PVA water-based glue and ensure the area is free from oil.
b) Sand back the glued area once it is dry using medium grade woodworking sandpaper.
c) Apply Extratec/ Fibreglass tape/ Edgetec/ Vellum.
6. STORAGE - To ensure that a bat remains in good condition, it is essential that the bat not be stored in excessively warm conditions (e.g. near to central heating). This will cause the bat to dry out and precipitate future problems. The bat should, ideally, be stored in a position that will allow a little moisture from the atmosphere to be absorbed.