How long can I be exposed to lead before I feel symptoms?

Depends. Just being 'exposed' to lead does not mean that you will develop symptoms. The lead has to be absorbed into the system in order to feel symptoms. For example, just having lead based paint on your walls does not necessarily mean that you will have symptoms. If the paint chips are put into the mouth (as kids sometimes may do), then the lead gets absorbed and symptoms can develop.
Heavy Metals Poison. You may never know that the way you feel is due to lead. It can make you very ill with nausea and vomitting if the levels are very high. But slow lead poisoning may not be apparent until testing. If you suspect lead exposure, have your physician check you.
Varies. If you think you have had lead exposure see your doctor and get a lead level checked, You may not recognize subtle symptoms.