It will depend on the type of tenancy agreement you have signed and what it actually says in your contract. Some landlords will try to evict tenants after a few weeks of not having received rent payment. Others may only wait a few days.
If you are unable to pay your rent, your landlord will issue you with an eviction notice. From that time you will have a period of up to 30 days to reply or make contact or pay the outstanding rent you owe. If you do not pay your rent within that time you will then receive a court order. If you lose your case and choose not to appeal you then have a further period of time before you can legally be evicted.
For more information on your specific case, have a look at your tenancy agreement and try to speak to your landlord to determine whether you can come to some sort of agreement. There may be local housing groups in your area where you can seek advice or you may even choose to seek legal advice from an attorney. There are a lot of websites offering free legal advice so have a look using an Internet search engine to find one that suited to your needs and circumstances.