I have a painful lump in my arm pit, how long should i monitor it?

It is a bit difficult to answer this question without a little more information about the lump, including whether it has appeared suddenly, how long you have had it and whether the lump is in the skin or under the skin. But I can try to give you some guidance on when it may be a good idea to see a doctor:
1) If the lump you have got is in the skin, then you will also see redness in the skin above the lump. This is usually due to an ingrown hair or a clogged sebaceous gland/sweat gland that becomes infected. This may go away on its own within a few days, but if you experience that the lump is just getting bigger and bigger, it may be a good idea to see a doctor who will have to assess whether you need antibiotic treatment or surgical drainage of the lump.
2) If the lump is under the skin, there will be no visible skin changes above the lump. If it has appeared in connection with menstruation, I would expect this to go away during menstruation or the following week. If it does not, however, I would recommend that you contact a doctor for an examination.
3) If the lump has occurred in connection with a respiratory infection and you have a fever, sore throat, decreased general condition and other signs of infection, I would recommend that you contact your doctor as soon as possible for further examination.

I hope this was helpful!