An individual who holds indefinite leave to remain (ILR), can legally settle in the UK without any time restrictions on their stay. Individuals from outside of the European Economic Area (EEA) who are under a Tier 2 (General) visa, must satisfy the requirements of Immigration Rules in order to obtain ILR.
Tier 2 visa on or after 6 April 2011
An Individual, who was granted a Tier 2 (General) visa on or after 6 April 2011, is limited to a maximum stay of six years (inclusive of any extensions). After this time, they will need to either apply and qualify for Indefinite Leave to Remain or leave the UK, the requirements you should meet are:
•    be paid an annual salary of at least £35,000 (income threshold set to increases from April 2018);
•    have Tier 2 (General) visa;
•    no criminal convictions;
•    have been living and working in the UK for five years; and 
•    spent no more than 180 days outside the UK in any 12 month period; and
•    still be required by their employer (sponsor) to continue their role.
A person working in a job role which falls under shortage occupation list are exempt from the annual salary threshold. 
Applicants will still need to meet all the other settlement criteria. We have provided http://www.visaandmigration.com/service/sponsorship-licence/82.html (here) to assist people to identify if they meet the requirement. 
Individuals granted a Tier 2 visa before 6 April 2011
A person who entered the UK before 6 April 2011, will not be subject to the six-year residency limit and can continue to apply for extensions to their visa indefinitely. Individuals in this situation will only be able to apply for settlement in the UK if they: 
•    meet the income threshold for ILR (details below), or
•    have been in the UK legally for 10 continuous years (known as long residence see here - http://www.visaandmigration.com/service/long-residence-ilr/25.html  ).
Income thresholds
(a) £35,000 if applying on or after 6 April 2016,
(b) £35,500 if applying on or after 6 April 2018,
(c) £35,800 if applying on or after 6 April 2019,
(d) £36,200 if applying on or after 6 April 2020,