I have multiple entry visit pass of Singapore valid for 2 year and normally they allow to stay for 30 days in a single visit. How many times I can do this if go out to my home country and come back again after 7 days? Is there any limitation on this ? Because it seems holder of the visit pass can stay in Singapore for more than 1.5 year if he/she use visit pass in this way.If we go by words it is multiple entry visa but, is it legal to do this or is it considered illegal?
Visas are issued for a specific purpose, and the type and duration of the visa issued are based on the purpose that you requested it for.
The actual conditions described on the visa form one part of the conditions and intent behind the visa, but they are not the only conditions that apply.
So in this case, the conditions on the visa are that you can stay for up to 30 days on each visit, and can have multiple visits. However such a visa would not have been issued with the intent that you could basically live in the country - if that was the expected intent then a different type of visa would have been issues.
Why this matters is related to one of the most misunderstood areas of visa - a visa does NOT give you the right to enter a country, even if you are fully applying by the conditions of the visa (stated conditions or otherwise). At the end of the day, the immigration officials at the border always have the last say as to whether you will be allowed into the country. If they believe that your visa has been obtained fraudulently (including mis-representing the time you intended to stay in the country), if they believe that you are not entering the country in line with the terms of your visa, if they believe you are entering the country to commit a crime, or for any other reason, they can deny you entry to the country at the border.
So whilst you may have a visa that states multiple entries up to 30 days, each time you re-enter the country the immigration official will look at your history of entries/exits from the country, and after a number of 30 days in/7 days out entries they will almost certainly begin to suspect that you are breaking the conditions of the visa - they might think that you are in the country to work, that you are residing there, or any number of other things that would be against the conditions of that type of visa. They might even simply suspect (correctly!) that your repeated short trips outside of the country are "visa runs" (trips with a sole purpose of extending visa validity) which are generally covered in immigration laws as not being allowed.
The second time you enter the country odds are this won't be a problem. But sometime after that - well before you make it even close to the 1 year mark - someone is going to spot the pattern, and put you on a plane back where you came from. Nobody is going to be able to tell you if that's going to occur on the 3rd visit or the 10th as it will depending on a number of factors including the exact type of visa, the particular immigration official, the answers you give to the questions they ask you, and countless other things - but it will happen eventually.
Immigration law is something it's best not to test the limits off. The results (ranging from cancellation of a visa to be banned from entry to a country for up to 10 years or more) just isn't worth the risk!