Is become a stream-enterer (sotāpanna) enough? I've quoted below from Ratana Sutta (The Discourse on the Jewels).


  Ye ariyasaccāni vibhāvayanti
  Gambhīrapaññena sudesitāni
  Kiñcāpi te honti bhusappamattā
  Na te bhavam atthamam ādiyanti
  Idam pi sanghe ratanam panītam
  Etena saccena suvatthi hotu


~ Cited from: Ratana Sutta (Pāli Text)

which means;  


  Those who clearly distinguish the noble truths,
  which have been well preached by the one with great wisdom,
  no matter how great they become in heedlessness
  still they do not take up an eighth existence—
  this excellent treasure is in the Sangha:
  by virtue of this truth may there be safety!


~ Cited from: Ratana Sutta | The Discourse on the Treasures

According to the above, stream-enterer will not have an eighth existence (re-appearing) no matter how great he/she become in heedlessness. Does it mean that a stream-enterer will become an Arahant within his/her next seven existences even without any effort?

If so, why would someone put an extra effort to become an Arahant after becoming a stream-enterer?
The Pali word translated as "existence" is "bhava". "Bhava" is a mental asava (defilement) and not another life. There are 10 fetters to break through for Arahantship. The stream-enterer has broken three fetters. Each fetter is a type of becoming. Therefore, the sutta says "they do not take up an eighth becoming" because there are only a maximum of 7 more fetters to break..  

In summary, it is pointless asking about stream-entry if you believe in reincarnation because this just perpetuates the "self-view" that must be broken for stream-entry. When there is the belief in reincarnation, it is the belief of "I will reincarnated or reborn seven more times". This is self-view. 

The stream-enterer, in the suttas, has the realisation of: "All that is subject to arising is subject to cessation". The realisation of the stream-enterer is not: "All that is subject to cessation is subject to re-arising".