How long is the average lifespan of a virus? I know this is hard to answer but asuming normal conditions (households etc)

Experiments with cold and flu viruses have shown potential survival times ranging from a few minutes to 48 hours or more.  They remain active longer on stainless steel, plastic and similar hard surfaces than on fabric and other soft surfaces. On any surface, though, flu viruses seem to live longer than cold viruses do. Other factors, such as the amount of virus deposited on a
surface and the temperature and humidity of the environment, also have
effects on how long cold and flu germs stay active outside the body.

I presume by lifespan you mean how long do viruses survive outside the host? Within the host some viruses such as HIV and the viruses that belong to the herpes family (viruses that cause cold sores and chickenpox) have a latent phase that can last for many years. When you get symptoms again this is known as reactivation.

In terms of outside the host it depends entirely on the environment the virus is in and also the size of the virus and importantly whether or not the virus has an outer membrane or envelope.

Non-enveloped viruses such as rhinoviruses, noroviruses and adenoviruses can in fact survive for long periods outside the host (up to several days) whereas viruses with membranes such as HIV, influenza and the herpes viruses which do well in the host survive for shorter time periods.

This is because many enveloped viruses rely on the proteins on the surface of the membrane to attach to the host cell, this envelope is generally sensitive to degradation to sunlight and normal cleaning procedures.

outbreaks associated with the non-enveloped viruses can continue for many months due to their relative stability in the environment, think of winter vomiting outbreaks, hand foot and mouth in animals as good examples of this phenomenon.