Usually you would put it in when you change tires/tubes. The way this stuff works (typically) is it remains liquid until forced by pressure through tire/tube and then sets like a silicone gel around the object to form a seal. The object puncturing the tire can stay in place for quite awhile and the tire/tube remains sealed, even if it moves slightly as any added deformation more liquid is converted to gel. Just be home when you pull the object out of the tire. For stuff like nails, thorns, screws, etc this kind of stuff works fine, it won't seal against really large objects like knife blades, concrete rebar, or .45 ACP's but then not much will.