Being without air conditioning for any amount of time in the summer in Texas is something we all want to avoid. When your system breaks down a good AC tech can get it going again, but when is it more cost effective to invest in a new system?

There are several factors to consider:

If your current AC system is 15+ years old then the components have already begun to degrade, and you are buying time until there will be a major component failure. While it is not necessary to replace the system due to age alone you should expect component failure is coming and budget ahead accordingly. Many homeowners with systems of this age choose to preemptively replace their AC systems so they are not left without AC during the heat of the summer.
If your current AC system is 10+ years old and a major component has failed it may be time to replace your AC system here too. If, for example, an outdoor unit (condenser) requires repairs that total half of the cost for a replacement of the complete outdoor unit then it begins to make sense to replace the unit. Repairing only the broken components will get you a 10-year parts warranty only on those replacement components. Replacing the whole unit will get you a 10-year parts warranty on everything.
If your current AC system is a builder's grade code minimum system, then odds are it is not very efficient, and your energy bill lets you know this every month. While the energy savings payback wouldn't make sense to replace a brand-new builder's grade AC system if your system is several years old and already starting to have maintenance issues then you may consider a new AC system to save on that energy bill.