If in a music piece crescendo is present but there are no decrescendos in the following measures, do I maintain the last loudness of the crescendo throughout until there is a decrescendo or another symbol of a dynamics?(e.g; The crescendo is a pianissimo - fortissimo; do I maintain the fortissimo until there is a change?)
Conceptually, yes: the end volume of a written dynamic is supposed to stick until further notice.
If you check with a loudness meter, you'll usually find that the actual dynamics both have in-phrase flexibility as well as being anticipatory of future dynamics, more often than not giving them more range to work with.
So if you have three crescendo phrases in a row, each will start and end louder than the previous one, but more likely than not each new phrase will not start exactly where the previous left off in volume but leave a bit more leeway for development by starting a someone quieter while still maintaining a continuity of tension/intensity.