The timing to check whether istihadha is little, medium or excessive, one must check at namaz time. Take a ball of cotton and place it on the surface of the vagina. If the blood is only on the surface of the cotton then it is little istihadha. If it penetrates the cotton then it's medium. If the cotton gets completely soaked with blood and flows out from it within a few seconds then it's excessive istihadha. With a tampon, it can stay for 1-2 hours hence one is unable to check with it while it's inserted. The above steps must be carried out to determine type of istihadha unless you know which type of istihadha it is in which case the above steps are not necessary.
Entering the zarih room and masjid is no problem for a person in istihadha however it is haram for a person in haydh.