Normally it is set as 70 to 75 % of the Total Physical Memory

To Change the Max Memory Settings

// First Turn on advanced options

EXEC  sp_configure'Show Advanced Options',1;
GO
RECONFIGURE;
GO

// Now we will set max server memory = 3500MB for the server

EXEC  sp_configure'max server memory (MB)',3500;
GO
RECONFIGURE;
GO

To check the configuration
EXEC sp_configure;