6 month visiting visa

Just wanted to ask do you think its necessary to specify on your invite letter when how long the person is going to come to you or could i just write example like this
I am writing to confirm I would like to invite my fiancée **** **** to visit me in the UK For a period of time We have now been together for 3 years
and intend to marry next year 2014 and we think it necessary for us to be together for a period of time and live together and see how are life would progress as if we were like married I would continue to work and take holidays and days off when **** is here
My fiancée has already had one 6 month visiting visa last year and came twice to England and stay for 2 weeks at a time but this time we would like for her to stay longer but she worry if we put down 3 or 4 month we will get refused does anyone have any feed back on this

codeone wrote:Just wanted to ask do you think its necessary to specify on your invite letter when how long the person is going to come to you or could i just write example like this
I am writing to confirm I would like to invite my fiancée **** **** to visit me in the UK For a period of time We have now been together for 3 years
and intend to marry next year 2014 and we think it necessary for us to be together for a period of time and live together and see how are life would progress as if we were like married I would continue to work and take holidays and days off when **** is here
My fiancée has already had one 6 month visiting visa last year and came twice to England and stay for 2 weeks at a time but this time we would like for her to stay longer but she worry if we put down 3 or 4 month we will get refused does anyone have any feed back on this

Well my fiancée and i apply for another 6 month visiting visa and we put on the letter as above and said we want her to come to the UK for 100 days and 10 days later visa was granted she coming at the end of the month happy days