Hi

.
Does anyone know how long my fiance has to have held his current job in order for me to qualify for the fiance visa?
He currently has a good job that meets the financial requirements, and he's been there 3 years. He is thinking of switching to another job that also pays well, but will this negatively affect our visa application? I don't want to have to wait longer because of this! Please help me! 

There are two ways of meeting the requirement using salaried or non-salaried employment:
Category A:- he has been with his current employer, earning at least £18,600 salary, for a minimum of 6 months
He will need to provide 6 months of payslips and corresponding bank statements showing a minimum pre-tax income of at least £1,550 each month (if the payslips vary they will only take the lowest payslip amount and multiply it by 12)
Category B:- 1) he has a current job paying at least £18,600 salary, but he has not been working there for 6 months yet
AND
- 2) he has earned a total of £18,600 (before tax) in the previous 12 months before applying for the visa
He will need to supply 12 months of payslips and bank statements showing a total income of at least £18,600 through the year.
So, it depends how much he has earned in total in the last 12 months as to whether he would be able to change jobs and still meet the requirements. If he won't be able to, I would recommend that he stays at his current job until after you have the visa, but if he can still meet the requirements after changing his job, then he could do that and you could apply under Category B instead.
See here for more information about the categories:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/525708/Appendix_FM_1_7_Financial_Requirement.pdf